The intro movie for Rock Band involves a band singing and keeping their balance on top of a moving car that appears to take turns at a near right angle on 2 wheels. The sheer power of rock kicks up debris in an old west ghost town the car flies through, and the band members fly off a cliff only to land on another vehicle.
Then there's the intro to Rock Band 2. Rock band duel on speeding cars, anyone? The one dude has a mic attached to a flail.
Guitar Hero II had an animated TV commercial in which a band plays "Woman" by Wolfmother. A giant asteroid headed straight for them is withered away to a pebble by the sheer power of their rock, capped with a Crowning Moment of Awesome when the guitarist catches it before slamming into the next phrase. The message appears to be that even an actual rock is no match for this trope.
Guitar Hero III has you battle the devil with, of course, "The Devil Went Down to Georgia". The intro has a guitarist literally climbing Mt. Olympus and defeating the gods of rock.
The intro movie of "Guitar Hero World Tour" has that same devil, this time in league with a Lawyer Friendly caricature of Kenny G (?!), defeated by an all-star rock band.
Lego Rock Band uses this in a number of game challenges. Including destroying a building, defending a castle, and jump starting a UFO. It runs on Rock-It fuel.
The plot of Guitar Hero: Warriors of Rock is to assemble the titular "Warriors of Rock" (characters from previous Guitar Hero games) and do battle with "the Beast" and recover the legendary ax-guitar that is the key to freeing "the Demigod of Rock". It's the literal interpretation of the trope.
Before Inis made Ouendan, they made Gitaroo Man for the PS2, a technicolour daydream about a superhero fighting aliens with his lightning-shooting guitar-weapon-thing. It was awesome.
Devil May Cry 3. Dante uses a guitar as a weapon to send electric bats flying at demons every time he shreds on it!
On that subject, in Gungrave: Overdose, you could play as Rocketbilly Redcadillac (no really, that's his name), a rockabilly ghost haunting an electricity-shooting guitar, who could fry enemies by playing it, even morphing it into an angel arm-esque beam cannon during one of his special attacks. Yeah, it's that kind of game.
In Michael Jacksons Moonwalker, based on the aforementioned movie, Michael could turn into a Mecha using Bubbles the Chimp as a power-up, and he defeated enemies by leading them in a dance sequence and by Moonwalking. In case you doubt us (and we wouldn't blame you, really) watch this.
In Final Fantasy IV, there is a dark elf that is powerful enough to cast a field that makes anyone wearing anything metal unable to move or fight, making your party nearly useless against him. The only way to break his spell? Is a song played by the original Spoony Bard himself.
And in Final Fantasy V, you have the choice of making all your characters into Bards.
Final Fantasy X-2 has the Songstress dress sphere, allowing the wearer to cast various spells through song or dance. The majority are buffs or status effects, but the most powerful abilities nullify magic attacks aimed at the party or lower the cost of all spells to 0.
Revolution X featured a more practical example of this trope by including a gun that shoots exploding CDs. Needless to say, it was So Bad, It's Good.
In Total Distortion, the entire plot is revolved around making rock music videos in a Dimension based on Rock. Combat with your evil Guitar Warrior enemies is also carried out via sonic blasts from electric guitars.
In the music-themed Guilty Gear series, where everything under the sun is named after or in reference to a rock band, artist, or song, the antagonist and part-time boss I-no uses an electric guitar as a weapon by both physically smacking her enemies around with it and using it to make a number of musical special attacks.
Link has used music to work magic ever since the series began, but in The Legend of Zelda: Majora's Mask he saves the universe from annihilation by summoning four gods with his ocarina. When he's wearing a Zora mask, he literally uses an electric guitar instead.
Brutal Legend has this trope at its core. Starring the aforementioned Jack Black as Eddie Riggs, a roadie for a pretentious nu-metal band who wishes he lived in a time where the music was actual heavy metal ("like, the early seventies"). He soon finds himself Trapped in Another World, which is practically based around this trope. As far as Eddie's concerned, he's in Ascended Fanboyheaven, at least until it gets worse. Examples:
The local Magic Music of healing is the beginning bass solo of Motörhead's Marching Off to War. First played by a Lemmy Kilmister expy voiced by the man himself, nonetheless.
Each of the five characters in Donkey Kong 64 has a musical instrument that can be used to instantly defeat all lesser enemies in the ape's vicinity. Naturally, this power comes up in the Final Exam Boss battle, although it's used more realistically as a way to get K. Rool's attention.
Also in Super Smash Bros. Brawl is the Assist Trophy Barbara, who attacks all opponents in the area with sonic waves from her guitar.
In Rayman 3: Hoodlum Havoc, Globox, having swallowed Andre, visits Dr. Otto to have him removed. Otto plays what sounds like a rock guitar solo using Globox's arm instead of a guitar but this has no effect. The second doctor they visit, Romeo, plays a drum solo on Globox's belly, but this doesn't work either. The third doctor Globox visits conducts a classical piece, performed by Otto and Romeo (again, using Globox as an instrument). This is too much for Andre, who finally leaves.
In EarthBound, the only way to pass through the tunnel to Threed is to befriend a blues band, who will give you a lift on their tour bus, allowing you to barrel through the tunnel full of ghosts while playing happy music, preventing said ghosts from grabbing you and throwing you back to Twoson. The band appears again to save you from the Clumsy Robot by jumping behind it and flipping the switch. Not exactly the power of rock but at least it's done by musicians.
Not to mention the main character's special attack is by default named "PSI Rockin." This is why it's very important to imagine Ness playing air guitar with his bat when you use it.
The Japan-only prequel MOTHER (AKA Earthbound Zero) takes this a step further. You actually have to use the sing command to defeat the Big Bad.
AND AGAIN in the Japan-only sequel MOTHER 3 the band DCMC sees through King P's false promises and helps you destroy him (and his little robots, too!) from the inside. Also, a member of your party played bass for the DCMC while donning an afro wig.
Also in MOTHER 3, if you tap the A button to the beat of the background music when attacking in battle, you can keep hitting the opponent in a combo up to sixteen times. Each playable character has a unique instrument (or bark, in Boney's case) that plays a note or two each time you hit when doing this. MOTHER certainly loves this trope...
If you play as Achmed Khan in Backyard Skateboarding, doing a Big Air Guitar Solo beats the final boss challenge and starts the concert.
Kira-Kira puts more emphasis on the mundane unifying potential that music has than the fantastical, but there's a humorous scene in Chie's route where Shika claims that one can manage movie-stunt-like feats while screaming ROCK! AND! ROLL!!! at the top of one's lungs. And then proceeds to jump from the back of a (speeding) pickup truck into its cab by doing so.
In Legend of Mana, playing the right music on specially enchanted instruments will charm the elementals into giving you shiny things. On the other hand, Harpies can cause naval disasters just by singing.
The Bard, in The Bard's Tale (2004), is able to summon creatures with his music, which can be played by everything from a lute to a singing sword, to an electric guitar axe.
The obscure PlayStation title Mad Maestro! did a fair job of invoking this with classical music.
For specific cases, one stage in the first Super Robot Wars Alpha allowed you to play the ending of the Do You Remember Love? movie, complete with the title song playing throughout the level. Granted, Minmay's music only triggers DYRL? plot points, but it a rare case of actually being in control of one big finale that was set to a giant love ballad.
In Alpha 3, you get to face Sharon Apple where her big song, Information High actually works against you in that you will be reduced to 50 morale (the lowest possible you can get) when you deploy AND have to slowly build up morale. Against multiple Ghost X-9's. Then, when it seems all hope is lost, out comes Basara Nekki and delivers his Catch Phrase: "Sharon Apple, LISTEN TO MY SONG!". All units jump up to maximum morale and proceed to rip apart the Ghost X-9's like no tomorrow.
Fire Bomber extends past this by exactly ONE boss in Alpha 3. This boss just happens to be Keisa Ephes, the final boss of the game and an amalgamation of dark emotions, wicked intent and all that stuff. Which is exactly why the Anima Spiritia of Fire Bomber is able to damage the monster, and even land the killing blow with them should the player wish. Yes, you can punch out Cthulhu with this trope.
In Song Summoner, you use songs from your iPod's library as catalysts to create "Tune Troopers" that battle the forces of evil for you. The Troopers level up when you listen to the songs used in their creation.
Alex's final weapon in The Colour Tuesday is her brother's guitar. It's required to defeat the final boss.
5pb from the Neptunia series uses her electric guitar as a weapon. Like Compa, her weapon is more suited for shooting magic bullets than dealing physical damage.
The rules of The Sims 2Self-Imposed Challenge the Apocalypse Challenge invoke this by linking the nuclear winter to the Music career path. It takes a Rock God to clear the skies and restore proper weather to the region. The flavor text has a short poem spelling out this trope:
Blast Mother Nature’s eardrums out; Show grandfather winter what you're about.
Make the existence look and nod; To the emergence of the Rock God.
With a mighty chord and a mighty roar; The icy grasp of winter is no more!
League of Legends champion Mordekaiser likes his weapons how he likes his music (Heavy and METAL). All of his abilities are a Shout Out to various heavy metal songs. If that's not enough, he's in a band with four other champions. He plays the Axe.
In IV, Rock 'n' Roll is one of the Wonders of the World, giving the civ that builds it access to Hit Singles, a luxury good that boosts your citizens' happiness.
V gives us the Great Musician in its Brave New World expansion. Great Musicians can create works of great music, which slowly generate culture points and tourism points, or they can be sent into rival territory to play a concert tour, which overwhelms their native culture with your own influence. The modern-day Great Musician looks like a three-person rock band. This power of rock may not be supernatural, but it's definitely an example of the true power of rock: winning hearts.