When it comes to annoying the Flash, he doesn't fiddle around.
"I remember every little thing as if it happened only yesterday. I was barely seventeen... and I once killed a boy with a Fender guitar. I don't remember if it was a Telecaster or a Stratocaster, but I do remember that it wasn't at all easy. It required the perfect combination of the right power chords and the precise angle from which to strike."
Suite Pretty Cure ♪ uses this a few times, first with Cure Muse (the masked version) conjuring a magical piano keyboard for some of her spells and later with Cure Beat's Love Guitar Rod, the magical guitar which turns into a battle ax and shoots exploding rings of magical death.
As a Spiritual Successor to the above, Senki Zesshou Symphogear has the leading cast, collectively called "Valkyries", as Magic Idol Singers fighting partially with their voice. During battle, they sing their Leitmotif in order to weaken enemies and fight properly against them. Even the background music changes actively to fit the theme of the situation. The "Assassin" portion comes as part of their Limit Break: a solo that creates a near-infinte amount of strength at the cost of their life.
Sphinx Pharaoh (harp), the two Lyra Orpheus and Benetnasch Mime (lyres, though Mime's armor can turn into a harp) and Siren Sorrento (flute) in Saint Seiya. In fact, Mime used to be pictured above. Nice Razor Floss, Mime.
Darker Than Black features an opera singer who can use resonance to destroy objects and stop hearts.
Midvalley the Horn Freak from Trigun uses the music from his saxophone to tear people apart. It also has a machine gun in it.
A flashback in the manga reveals that he used to be part of an entire band of Musical Assassins. Their modus operandi was to take out the entire audience to hit their target.
Tayuya from Naruto has a flute that can make illusions and control summoned demons.
Shoutmon of Digimon Xros Wars is themed around rock music, so most of his attacks are named after types of rock, he uses a microphone as a staff weapon, and his Soul Crusher attack turns his singing into his microphone into a blast of fire.
Amy/Sailor Mercury plays a harp to perform her Mercury Aqua Rhapsody, which attacks the enemy with water.
Michiru Kaiou/Sailor Neptune can use her violin to attack her enemies in the manga version of Sailor Moon.
Ali from the Makaijuu filler arc of the first anime uses his flute to awaken the Cardians and give them instructions. However, this has its limits - when An isn't around to pick out a Cardian for him, Ali tries to summon one randomly and accidentally summons a Cardian that doesn't obey him.
In the live-action Pretty Guardian Sailor Moon, Zoicite uses the piano to compose attack songs and then, by playing the completed piece, creates various effects, including summoning a youma, allowing an ally to teleport, and long-distance targeting of various people and objects (e.g., princess-related, including a visitor to Tokyo, a picture-book, and a girl who merely identified too strongly with the princess in a storybook) with effects that range from unexplained sickness to Spontaneous Combustion. Good thing the Big Bad is patient, because Zoicite sure doesn't rush the process.
In Mononoke one of the monsters uses a biwa (Japanese lute) to inflict dangerous hallucinations on the unlucky humans it comes across.
In Katekyo Hitman Reborn!, one minor villain - M.M. - uses a clarinet that functions as a microwave, by using sound vibration to agitate water molecules and thus rapidly heating objects.
Neon in Flame of Recca uses a Madogu in the form of a flute that manipulates sound waves.
Kidaf Gillot from Scrapped Princess used an instrument to command robotic bugs for assassination.
The finale of the first Lupin III manga series has Lupin up against a conductor whose assassins and explosives are in sync with his music.
A later Lupin III (Red Jacket) episode would have a conductor who could control the minds of people listening to his music by waving his baton.
Shiho Munakata's Element in Mai-HiME is a flute that lets her control sound waves and give orders to her CHILD, Yatagarasu.
Lena from Shamanic Princess plays a flute to control vine-like magic tendrils. She doesn't usually attack with them directly, though she can do so in certain circumstances.
GaoGaiGar's Mic Sounders the 13th (and his 12 preceding brothers) has music-based powers. He can use Disc M to cancel out an enemy mech's abilities, use Disc P to empower other GS-Ride-based mecha, and use Disc X to destroy things. In fact, the reason Disc X isn't used to destroy every enemy mecha that shows up is that, unless it is precisely tuned to perfectly match whatever he's dealing with, he runs the risk of destroying everything with it, including the Earth.
The Winged Ones in Pure sing as well with various effects. Lady Bat can put his victims to sleep, Lanhua can split into twelve mini-musicians and force her listeners to dance until they die, and Alala can shoot stars that strike her targets.
A villain in Speed Grapher would count as one of these; when he used his powers, his body essentially turned into a speaker system.
Fou Lafine of Bt X, whose Weapon of Choice is a violin. Unlike Kurumada's examples in Saint Seiya, disabling X's auditory sensors did block the attack. Unfortunately Fou was no slouch in combat either.
Early on in R.O.D. The TV, the Paper Sisters face off against an enemy whose weapon of choice is... a huge pipe-organ. Which projects walls of sound that breaks down their Papermaster powers, and just generally knocks them around. (It can also conduct psychological warfare by using 'infrasound' to distract and stress out the intruders without them realizing.)
Let's not forget about Beethoven's suicide orchestra in the Read or Die OVA.
Bleach has the Visored Rojuro "Rose" Otoribashi, who can kill you by playing his electric guitar.
One Piece has Scratchmen Apoo. What's his instrument? His own body - he has a Devil Fruit that turns his body parts into instruments. He is a living orchestra. What can he do with his music? How about cymbal crashes that dismember people, and drumbeats that cause explosions?
A somewhat lesser example is Brook of the Straw Hats. Prior to the Time Skip, his music only served as an accompaniment (no pun intended) to his fighting style, creating his trademark. He could also use it to put enemies to sleep. But two years later, he's tapped into the full power of his Devil Fruit, so he can use this trope to become a Master of Illusion.
Soul of Soul Eater uses his skills as a pianist and general musical knowledge in order to, well, orchestrate the black blood and control his own insanity to increasingly devastating effect.
Also, when he and Maka were fighting Arachne, he was able to use her own spider webs to transmit Maka's anti-demonic wavelength into everyone to prevent them having their heads torn apart and also to injure Arachne herself.
In Hunter × Hunter, Bonolenof plays music that either summons or creates weapons and fighters at his disposal. His body is punctured with holes and reeds making him a living woodwind instrument, so he's also a Dance Battler as this is how he plays himself.
Fairy Tail has two. Vidaldas, a freakish rocker modeled after a KISS member, who takes control of people with music and makes them fight, and Lullaby, a sentient, soul eating flute that devours the souls of those who hear it's melody... and also turns into a giant wooden demon.
General Cross also uses one, or more specifically his second weapon, The Grave of Marie, does. Marie was an exorcist whose voice could paralyze every Akuma who hears it regardless of how strong they are. Cross "repurposed" her after her death, making him even more powerful than he already is.
The Flash villains the Fiddler and the Pied Piper can wreak havoc with a violin and a flute, respectively. In the Countdown to Final Crisis series, Pied Piper even destroyed a Brother Eye/Apokolips hybrid with Queen's "The Show Must Go On." Probably one of the best examples of a Musical Assassin ever.
The Fiddler had a female successor named Virtuosa whom has only ever appeared in books featuring the Secret Six, which is fitting since she was created by Gail Simone. Virtuosa claims that she's an ardent admirer of the original Fiddler and took up his violin and legacy after his death in honor of him. She in fact had something of a fan girl moment when Fiddler was revived as a Black Lantern during Blackest Night and offered his violin back, though he was more interested in ripping her heart out.
The Piper in Adventures In The Rifle Brigade is a Scottish bagpipe player of over 100 years of age whose talent with the instrument (his current set was crafted from the flesh of the fellow who broke his last one) is unnatural to the point of tipping the scale all he way back into unbearable. Men have torn their own heads off to escape his music.
One Usagi Yojimbo story featured a series of killings with a flute-playing ghost as the primary suspect. It wasn't him.
Johnny Guitar and Dr. Sax in Avengers: The Initiative can level entire buildings with their guitar and saxophone, respectively... but they've been losers as supervillains, being passed over for every possible team-up because they were either too gimmicky, not mutants, or not gimmicky enough.
Dr. Crescendo in Tomorrow Stories' Greyshirt happens to be a Mad Scientist in addition to a violinist - and attempts to use his sinister music to prove his long-mocked theories true.
The Pied Piper of Hamelin: The 1440s German legend about a rat-catcher who, back in 1284, freed the city Hamelin (Hameln in German) from rats by luring them with magical pipes into the Weser River. When the townspeople wouldn't pay him for his services he used it to lure their children into a cave, from which they were never seen again.
Used in Manchester Lost, as a cross between this and the Power of Rock. Beating up Satan with nothing but Don't Stop Believing by Journey, albeit performed by Archangels.
In The Graduation Class, a Discworld fanfic in which the author has several characters who fall into the Ascended Extra category, after their names are given in canon, one with a very slight elaboration. And they are all Assassins. This trope was used by Emmanuelle les Deux Epées from Quirm who kills someone by playing a certain set of cords on a violin, firing a crossbow bolt. The author chose a violin deliberately, as the character was working as hitperson for the Troll crimelord Chrysoprase, to spoof 1930's tropes of the killer taking an unexpected tommy gun out of a violin case to play the music with.
The blind musicians (well, only one of them was blind) in Kung Fu Hustle can tear people apart with their music, in the form of invisible blades and fists, and at the height of their performance they're summoning hordes of Chinese zombie warrior ghosts out of their instruments.
Wild Zero features real-life Japanese rock band Guitar Wolf fighting hordes of aliens and zombies with homing guitar picks, samurai swords in the necks of their guitars and the pure POWER OF ROCK AND ROLL.
In Scott Pilgrim vs. The World a Battle of the Bands turns literal, with Sex Bob-omb and the Katayanagi Twins blasting each other with waves of sound from their amps. Eventually those sound waves turn into a giant gorilla and a giant, two-headed dragon and fight it out over the heads of the audience.
Lets not forget the bass battle between Scott and Todd where Scott gets blown through three walls by the power of Todd's bass. He winds up landing in a coffee shop where he tricks Todd into drinking half and half.
In The Punisher (2004), Frank has to face a hitman by the name of Harry Heck in one scene. He first sees Heck in a diner as the man sings and plays what appears to be an original song and is then informed, "I'm gonna play it at your funeral". He waits until the vigilante leaves the diner before trying to kill him and never incorporates his guitar in his actual killing.
In Isaac Asimov's Foundation series, the character of The Mule has psychic abilities that can essentially destroy people's mentalities, often aided with an instrument called a Visi-Sonor. He can "aim" the music to cause life-threatening depression, and does.
In Alan Dean Foster's Spellsinger series, the main character is a man sucked from his own world into one where animals are big and can talk. There, he finds out that he can do magic by singing about what he wants. God of Thunder would probably result in an awesome incendiary finish to his enemies. Guess what Pink Floyd's Money generates.
In Simon R. Green's Deathstalker books, there's the rogue Investigator Topaz, of the "Siren" class of esper, who kills by singing. She took out the five hundred men chasing her with one song.
In particular, Norman Spinrad's Jerry Cornelius short story "The Last Hurrah of the Golden Horde" arms Jerry with an electric violin with built-in amps which made the high notes ultrasonic, the low notes infrasonic, and the audible frequencies suppressed. When Jerry pops in his earplugs and plays various songs (ranging from "Wipeout" to "God Save the Queen") weird things happen to the minds of his audience.
Seven hundred chanting kindergarten children achieved satori and began to eat a huge American flag drenched in soy sauce.
In Terry Pratchett's Discworld each Nac Mac Feegle clan has a bard known as a Gonnagle whose music/poetry is so terrible that it can be used as a weapon. Fortunately for the Feegles, the Discworld has never heard of the Geneva Convention, since Gonnagle songs would likely be classified as war crimes under it.
The Llamedosians had terrible feared battle-choirs whose singing could decimate enemies and win wars. (A Shout-Out to Film/ Zulu). They also had battle-harps whose tones could cause walls to fall and enemies to embarrass themselves.
And Death can kill all of reality with a single, well-played strum.
Michelle West's Kallandras is an assassin who happens to be "bard-born". He can make people feel emotions through music, and can speak in a voice that makes anyone who hears it obey, although he doesn't like to use it.
In the Old Kingdom series, the Abhorsen uses seven bells to control and banish undead. Each bell has a different power.
Sleeper, Waker, Walker, Speaker, Thinker, Binder, Weeper. AKA Ranna, Mosrael, Kibeth, Dyrim, Belgaer, Saraneth and Astarael. If you hear Ranna, you'll just go to sleep. If you hear Astarael, the most powerful bell, you and the wielder will be hurled deep into Death.
In the Kingdoms of Thorn and Bone series, music written in certain modes can have a powerfully harmful effect on anyone who hears it- fatal, if played correctly. A historical Evil Overlord and Evil Sorcerer, the Black Jester, was noted for assembling orchestras and choirs for this precise purpose, so skilled they could wipe out whole cities; Prince Robert in the present storyline is also fond of this technique, with the added bonus that he's immune because he's actually undead.
The 'death flutes' of the Zoromen in the Doc Savage novel Murder Melody can induce either unconsciousness or death in the listener depending on the tune played.
In Peter and Max, a spin-off novel of the Fables comic series, you have Max. His flute is imbued with enormous eldritch power, so while he wreak large-scale havoc like disease and mind control, he can also kill people individually just by playing his flute.
Possibly played for laughs and possibly played really, really weirdly straight in Libba Bray's Going Bovine with the trumpet battles between Junior Webster and the Wizard of Reckoning, with a jazz battle literally being a final face-off between the two.
In Jayne Anne Krentz's Arcane Society series about various types of psychic talents, book 5, titled Running Hot, features a Siren, a singer who channels psychic energy through her voice. She can lightly mesmerize a large audience with lower notes, but needs to be able to sing a D, E, F, or higher to focus her song on killing a person. The Siren in question is a trained opera singer who takes out enemies of her wealthy, well-connected family.
In Stephanie Draven's Siren Song, the protagonist is an Enthralling Siren who can control men by singing. Although the heroine does not want to harm anyone, most sirens are described as serial-killer-types who compel men to drown themselves and enjoy the feeling of power it gives them. The villain is also a siren, and by singing, can even compel a man to shoot himself with his own gun.
In Mercedes Lackey's Bardic Voices, Rune and Sparrowhawk get trapped "underhill" by elves who force them to play for them. Rune turns the tables on them by using her fiddle to force them to dance until they drop from exhaustion, then starts playing laments to make them cry until they nearly die from depression. She threatens to keep on playing until they DO die unless they release the two of them. In her "Valdemar" books, many bards have projective empathy that can make their audiences feel the emotions of their music; although we never see any of them use it to harm or kill anyone, it is implied that a Bard with a strong enough Gift could do so.
Doctor Who: An improvised musical weapon features in "The Lazarus Experiment", when the Doctor uses a church organ to kill the Monster of the Week, who is stretching his mass very thin.
An Classic Who episode, "Revelation of the Daleks", featured a cannon that fired compressed rock and roll.
Musical weapons are the gimmick of Kamen Rider Hibiki, where they use the ultrasonic vibrations of their music to shatter the Monster of the Week. The main Rider uses drumsticks (creating a drum on the surface of the monster), the second Rider uses a trumpet-slash-machine gun and the third Rider uses an electric guitar. All three styles are apparently relatively common among the various Oni. Another present-day Rider uses a harp, while four movie-exclusive Riders from the distant past use a flute/machine gun, a gong stuck, a triangle and a pair of cymbals.
Decade and Diend get in on the act during the finale of the Hibiki arc of their show, each using a set of drumsticks (joined by several other Riders using trumpets and guitars) against the final monster there.
British sketch show That Mitchell and Webb Look featured one skit with a green clarinet that made its targets sing and dance an embarrassing truth about themselves. It could only be countered by a red tuba that makes you shit yourself.
Lorne from Angel has a tendency to emit ear-piercing and glass-shattering shrieks whenever he finds himself in danger. Also, regular singing apparently causes great pain to others of his kind.
In The Middleman, there's a cursed tuba that anyone who hears it play will "drown in the icy waters of the North Atlantic." Naturally, there's an immortal guy who wants to play it to a large crowd.
In Heroes volume 5, Emma starts out as just a deaf woman able to see sound as colors. Later, she discovers the true powers of her ability - she can focus the sounds that only she can see into a devastating shockwave, and she can concentrate on people while she plays music to force them to come to her.
In the Friday The 13th: The Series "The Maestro", the cursed antique of the week was a music box who's melody forced the listener to dance themselves to death. Death occurred either though strain on the body or dancing into deadly situations, such as out a window.
"The Lyre of Orpheus" by Nick Cave and the Bad Seeds:
In the Swedish legend of the Hårgadance a mysterious fiddler turned up one day in the village of Hårga during a celebration and started playing a song on his violin that made everyone dance enthusiastically. Too late, the people realized that the man, who they now saw had a goat's leg, was the Devil himself. They were unable to stop dancing and kept doing so while the fiddler led them up the nearby Hårgamountain. There, he kept playing until everyone had been killed and their bodies torn to bits by the intense dancing. The top of Hårgamountain is still flat to this day because of the wild dance.
In the Finnish national epic Kalevala, Väinämöinen, a wise old man with the gift for magical singing. engages in a battle in the said craft with Joukahainen. Väinämöinen almost sank Joukahainen in a bog with his song, but decided to take Joukahainen's sister instead.
The harp of the god Angus Og, in Irish mythology, held great power. It was captured in battle by the hideous troll race of the Fomorians. One night, Angus and a comrade crept into the Fomorian camp and were discovered stealing the harp back. Undeterred, Angus played the harp and even the foul trolls were overcome by the beauty of his playing. Legend has it he played the harp three times. first he played the notes of joy that caused the enemy to rejoice. Then he played the notes of sorrow that caused them to be overborne with tears. Finally he played the notes of sleep, that rendered them insensible. Thus was he able to return unscathed to the Tuatha de Danaan having won back the harp.
In the Warhammer 40,000 setting, 2nd edition Slaaneshi Noise Marines use sonic weapons whose destructive sound waves can tear humans and even tanks to shreds and looked like electric guitars. It's not so much 'music' as 'flesh-rendingly loud noise', and were replaced with less fitting guns.
There were also troops belonging to the Ork Goffs clan that wielded electric guitars. Their name? Goff Rockers, of course.
In the background novels of the "Horus Heresy" series, the Space Marines encounter Warsingers, leaders of a xeno race who use their song (literally) to kill even Astartes with lethal swiftness. Their song was described as serenely intoxicating.
Lets not forget Eldar Bonesingers, who use music to shape 'psychoreactive' wraithbone into a variety of weapons and tools. Their songs may not be literally lethal, but the gravitic induction shuriken launcher they just sang out of a lump of plastic is.
Exalted wouldn't be a proper anime-based RPG if it didn't have one of these (it's a Dragon-blooded Performance charm, natch). The Abyssals also have a Performance charm that can deal damage, and there's a Celestial martial arts style based on this trope: the Silver-Voiced Nightingale Style.
The Bard from Dungeons & Dragons can hypnotize people with a musical performance, and there are prestige classes in Third Edition that will let him summon fire or lightning with the sound of music. Bardic songs are also good for debuffs.
Not to mention multi-classing into Rogue or Assassin in 3e/3.5/Pathfinder. Then it's a bit more literal. Fascinate and Death attack can be a killer combo.
Weapons Of The Gods has a martial arts style called Music of War which lets you attack and debuff enemies from a distance by playing a musical instrument.
B-Flat Bart from Peter Schickele's Hornsmoke: A Horse Opera, who appears to mow down the rest of the cast (all of them musicians) with his trumpet.
The Bard class from the various Final Fantasy games uses a variety of instruments, the most popular of which is a trusty harp. Sometimes they induce Standard Status Effects, but the chances of doing so are very low. Because of this, they're often also a Spoony Bard.
In Final Fantasy V songs are extremely effective, when used right. One of them causes massive damage to undead, for example. Another can consistently cause confusion to the right kind of enemies.
Final Fantasy IX plays with this. Eiko's weapon class is a flute that she hits people with it for her standard attack. The real reason she has it is to do Summon Magic. She's a White Magician Girl.
Also subverted in this manner in Final Fantasy Tactics Advance and Final Fantasy Tactics A2, where the Beastmaster, Animist, and Bard classes all equip musical instruments such as flutes and harps to learn appropriately musical and monster-based skills (Music Soothes the Savage Beast, get it?), but when used in an attack, the unit in question simply brains the enemy with it (Kabong!). Suddenly, the descriptor from Final Fantasy Tactics A2, "Ill-tuned Instruments", makes much more sense.
The original Final Fantasy Tactics (subtitled War of the Lions because of the GBA sequels) also featured bards. Their songs either affected every ally with a positive or every enemy with a negative effect.
Ironically he is also one of the least cruel, laziest, and nicest Organization XIII members, so not only will you hate him for being a hard boss, you will most likely feel immensely guilty for erasing him from existence afterwards.
Not to mention he seems to be more interested in playing than actually attacking you. One of the reaction command moments in the fight has him playing away, seemingly completely oblivious to Sora, who looks like he is either preparing an attack or trying to get his attention
I-No from Guilty Gear is armed with a guitar and knows how to use it. Explosive notes are the weakest of her weapons. Her ultimate attack, Megalomania, projects horrifically powerful bursts of sound from a pair of miniature amplifiers.
Yurika Kirishima from the Rival Schools sequel Project Justice is a subdued variant, her signature weapon being a violin.
Lord of the Rings Online: Not an assassin per se, but the Minstrel class uses instruments and voice to deal respectable damage.
Vulturon the Condoroid of Mega Man ZX Advent is capable of repulsing melee attacks (and even some ranged attacks), discharging waves of scrap parts, and animating defunct Mechaniloids. While these do sound like ordinary powers to many a medium, what makes it noteworthy is that he does it all...with power chords. He also occasionally simply swats you with his guitar. (His Japanese name is Condorrock the Vulturoid. Go figure.)
Tech Kraken of Mega Man Zero 4 did something of the sort. At least, one of his attack calls is "Sound...dance!"
In Gungrave: Overdose, the rockabilly Rocketbilly Redcadillac (yes, that's his name) implies that he was once one of these assassin types. Now, though, he's The Atoner and a ghost, possessing an electricity-shooting guitar.
In Devil May Cry 3, Nevan, a succubus boss character, becomes an electric guitar after you defeat her as Dante. She is arguably the most difficult to use yet most powerful weapon in the game, easily allowing SSStylish combos if used properly. It electrocutes enemies by shooting lightning at them and summoning bats to attack them.
Command & Conquer 3 has the sonic emitter as the GDI's high tier defensive structure, which is basically a gigantic loudspeaker. In the expansion, the weaker but mobile version is also available, known as the Shatterer.
Tiberian Sun also has the sonic tank, which is the prototype of the above-mentioned weapon.
Zhen Ji in Dynasty Warriors (except the sixth installment) uses flutes for sound waves. In 7, she is joined by Cai Wenji, who uses a harp.
Sister series Samurai Warriors has Motochika Chōsokabe, who wields a shamisen (a Japanese guitar, more or less).
Super Smash Bros.. Brawl has several. Donkey Kong uses bongos for his final smash (the damage and range of which depend on your rhythm); an assist trophy plays guitar; Jigglypuff and Peach put enemies to sleep with song; Luigi uses a dance number to summon the Negative Zone; and King Dedede has a song and dance summonBig Gay Dance.
The Prismriver Sisters, a trio with each one associated with a different instrument. In the ninth game, their EX attacks involve them sending over their instrument to your side and having it launch bullets at you.
The fourteenth game adds the Tsukumo Sisters and Raiko Horikawa, each with a different of their own.
Chrono Cross had Nikki. His weapon was, of course, a guitar (the guitar pick, if you wanna get technical), and his basic attack was playing it. His first special was to bludgeon someone with it, Hendrix-style.
Irenes did something similar with her harp. And of course, there's bunnygirl Jan, who beats on a drum with a carrot.
God Hand: one of the bosses is a guitarist. With lightning projectiles. And he has a return battle where he pulls in a drummer, too.
Lyude from Baten Kaitos: Eternal Wings and the Lost Ocean, used a type of weapon called a "sound shock gun", which was something of a cross between a trumpet (or other instrument) and a machine gun. Oh, and all of his finishing moves were musical terms.
A particularly conspicuous card for Lyude: "Assault Trumpet". Description: "Designed for covert use by assassins."
In Donkey Kong 64, each character could obtain a musical instrument that could be used as a limited-ammo special attack to destroy nearby enemies (Donkey Kong got bongos, possibly inspiring his final smash in Super Smash Bros. Brawl).
Sona, the Maven of the Strings from League of Legends attacks by firing waves of sound from a guzheng-like instrument. She can also amplify her attacks by playing songs, send out discordant waves of sound to hunt down opponents, and her ultimate ability forces opponents to dance to her song while their health drains.
Cornet from Rhapsody: A Musical Adventure uses her horn as a weapon, although it's more often the channel through which she controls her helpful puppet/monster friends.
Monster Hunter games of late have adopted a Hammer subclass called the Hunting Horn, which induces Standard Status Effects by entering what's called Recital Mode, and then pressing attack buttons to play different notes. Different note combinations got you different effects. In a nice change of pace from your standard self-buffing classes in an action-RPG, they're also good for clocking a monster over the head without a Recital beforehand.
Pokémon. Sure, all that Sing does is put the opponent to sleep, but there's always Perish Song, which kills anyone who hears it within three turns. Not to mention Hyper Voice, Bug Buzz, and other sound-based attacks. And then there's Meloetta. This Legendary uses mostly sound-and-music-based moves.
There's enough sound-based moves to warrant the Soundproof ability (which blocks them outright).
Cocoron, a Japanese only video game, in which you build six characters to play as, including the weapon they use. One of the weapons is musical notes, and the characters generally launch their attacks from their heads. DeceasedCrab, Youtube celebrity, made this famous through his Let's Play of the game, with the third character he made being Tasian, the Singing Ninja Tank. .
Ephemeral Fantasia may as well be called Lute Hero: The RPG as a lot of story segments require you to play along, for serenading your girls, to annoying the main villain. Sadly, in normal combat, you just pull a sword from the spine. Music instead takes the place of healing and support magic, and the occasional song-related attack spell. The hero plays these automatically though.
In the original campaign, Bards have a variety of songs. While most of it is just buffs or debuffs, the ultimate song, Hymn of Requiem, is a very nasty attack that deals sonic damage (which is unavoidable) that is twice your Perform skill to either one enemy, or splits it across multiple enemies...and then heals your whole party by the amount of damage dealt. It does this every turn for five turns (ten if you took the Lingering Song feat). Spoony Bard the Player Character is not.
One of the bonus weapons in Castlevania: Curse of Darkness is a badass looking skull guitar that would make metalheads envious. Hector attacks by RAWKIN' OUT DOOD! The screen shakes and vibrates for every uninterrupted riff he plays, though some larger enemies aren't staggered by the playing, ditto for those which can move underground or 'phase out.'
In Luminous Arc 2, the Brilliant Witch, Dia, uses a conducting baton as a weapon. Kaph, who later joins the party, subverts this trope with a guitar...that fires bullets (one of his best weapons for illustrating this trope is the Keyguntar, which is one-third keyboard, one-third gun, and one-third guitar). Also, Sadie uses horn as her weapon, and actually has a spell to emphasize on that fact.
Amusingly, Dia is rumored to be tone deaf, and Sadie has to play badly to do any damage - she can play well, but that only stirs up a light breeze. Meanwhile, the guy who doesn't actually play an instrument on the field is a multi-talented and highly respected artisan.
A newly-added class in Atlantica Online uses a guitar as a weapon. Despite very obviously being an acoustic guitar, it makes electric guitar noises, likely because of Rule of Cool. The same reason there's a class that uses what appear to be modern chainsaws in an apparent pre-20th-century Anachronism Stew setting — there were Chainsaws in the 19th century, but they don't look anything like what a modern viewer would expect.
The whole battle system of Mother 3 revolves around Rhythm Combos. If you select your basic attack and press the A button to the beat of the music, you can link together combos. Almost every enemy has their own special song to combo to.
Also, whenever you do hit the enemy, each playable character has a different kind of musical note played upon attacking. (Flint = Saxophone, Duster = Bass, etc.)
New Fassad (and Miracle Fassad) also counts, using off-key music as an attack.
A mod for Doom lets you use the Rick Roller, a stereo that plays "Never Gonna Give You Up" and deals more damage than the BFG. Not even doom monsters can withstand Rick Roll!
One SpongeBob SquarePants game allows him to play a guitar riff so awesome that its sound is a guided missile.
The entire plot (such as it is) of LEGO Rock Band is that your band uses the power of music to achieve everything from fighting off a monstrous squid to obliterating an asteroid on it's way to hit the planet.
The Nightmare Before Christmas: Oogie's Revenge has an attack that is nothing more than Jack singing and dancing, but shoots musical notes at the enemy that are devastating compared to Jack's normal attacks. In fact, this is the only way you can defeat the final boss (and is even powerful enough to summon giant balls of snow to rain from the sky and fire to shoot from the ground).
As the above quote suggests, Brutal Legend loves this trope. Three people have electric guitars that they can play to use magical attacks: Eddie Riggs, Drowned Ophelia, and Emperor Doviculus. Also, the Drowning Doom's Organist unit plays a funeral dirge that weakens enemy troops while Ironheade's Thunderhog plays bass riffs to heal wounded allies, and General Lionwhyte's metal screams can shatter glass and mirrors.
Also, the Roadies of Ironhead can destroy enemy buildings by blasting them with sound waves from the giant speakers they carry, and the Rock Crusher buffs friendly units with the Power of Rock.
Darkstalkers has Lord Raptor/Zabel Zarock, who has a guitar counter-attack, in addition to looking like a zombie rocker.
From the same game, Lilith also has a special, the Gloomy Puppet Show. When the move input is completed, she tosses a top hat at the opponent. If it connects, she switches outfit, while the opponent is put upon a makeshift stage. A mini-game then starts, where you need to tap the right keys in the right rhythm. Depending on how well you perform, the opponent could take massive amounts of damage, or very little.
In Tales of the Abyss, Tear Grants makes her first appearance by singing all the guards in Luke's house to sleep so she can get in and assassinate Van. All of her highest level fonic artes, including a couple of very powerful magical attacks, are cast by singing the Yulian Fonic Hymns.
In Soul Symphony, Protagonist Olivia's weapon of choice against monsters is a clarinet allows her to fly and create magical shields, among other things. Once John joins her in the fray, he uses a trombone as a sword and a cannon.
In Movement 2, the giant Gnarlequin that acts as the 'boss' of Charlie's soul attacks John and Olivia with soundwaves from a giant electric guitar.
In Movement 4, Ashley's embodiment of Tom Mustaine in her Soul World also attacks with electric guitar strums.
Yellow Submarine: "They hate music that much, do they?" "They shrink at the very sound!"
"Music Land", a 1935 Disney Silly Symphonies short, features a Romeo and Juliet-esque romance between the saxophone prince of the Isle of Jazz and the cello princess of the Land of Symphony, and briefly leads to a literal musical war between the parents of the couple.
Bluegrass and Melodia are opposing musical assassins on SilverHawks.
Soundwave from Transformers Animated. He uses Laserbeak and Ratbat as a powerful electric guitar and keytar respectively.
An interesting twist is the old Looney Tunes cartoon "Now Hear This", in which a man finds a 'horn' which creates strange and magical (and which would be violent in other circumstances) occurrences throughout the 'toon. In the end it turns out to have been the devil's horn! He screws it back onto his head and explodes away, leaving the man in confusion.
Note that The Jammer is considered to be the most lethal weapon on Megas. It has more systems preventing its use than anything else.
Batman: The Brave and the Bold has fan-favorite Music Meister who uses his singing voice to hypnotize his victims. He doesn't need to kill people with his voice, as he can easily order them to kill themselves by dancing into a rocket blast.
In G.I. Joe, Metal-Head was capable of destroying a skyscraper with the amplified sound of his metal music.
Variant: Quick Draw McGraw's alter-ego El Kabong uses a guitar as his weapon. KABONG!
The 1929 Krazy Kat short Ratskin (the first one with sound) features an Indian shooting arrows at Krazy. He retaliates by using the arrows to create a makeshift harp, playing it to produce musical notes and firing them at the Indian.
Midgard of the Counterfactual.net crew from AH.com: The Series, who once destroyed a civilisation with the right power chord.
In Super Stories, the Host weakens and hurts his opponents with music. His partner, Cabaret, is immune.
In the Corridor Digital video Dubstep Guns this trope is combined with some lasers for interestingly devastating results.
Siren from Phaeton can manipulate both people and the universe with music, she also has a weapon/instrument that doubles as an instrument/weapon. No one is entirely sure which it is and which it doubles as though.
There is a sonic projector called LRAD which can put out incapacitating noise up to 1000 feet from the device.
"At Woodstock in 1969, [Abbie] Hoffman reportedly interrupted The Who's performance to attempt to speak against the jailing of John Sinclair of the White Panther Party. He grabbed a microphone and yelled, "I think this is a pile of shit while John Sinclair rots in prison ..." Pete Townshend was adjusting his amplifier between songs and turned to look at Hoffman over his right shoulder. Townshend reportedly ran at Hoffman with his guitar and hit Hoffman in the back, although Townshend later denied attacking Hoffman. Townshend later said that while he actually agreed with Hoffman on Sinclair's imprisonment, he would have knocked him offstage regardless of the content of his message, given that Hoffman had violated the "sanctity of the stage," i.e., the right of the band to perform uninterrupted by distractions not relevant to the actual show. The incident took place during a camera change, and was not captured on film. The audio of this incident, however, can be heard on The Who's box set, Thirty Years of Maximum R&B (Disc 2, Track 20, 'Abbie Hoffman Incident')."