Derek: But I won’t do it, I won’t kill anybody.
JP: It's not up to you. At the proper moment they’ll trigger you. Usually using some kind of auditory or visual Pavlovian response mechanism.
A specific musical cue—not in the BGM, but in actual music being played by someone or something In-Universe—serves as the trigger for some event. The more dramatic the event, the more likely that the trigger occurs at the end of the song, in which case the song functions as a melodic (and quantum-mechanical-anomaly-proof) form of the Dramatic Countdown Clock.
Diabolical Masterminds are particularly prone to incorporating music into their master plan. Maybe manipulating a public performance to their own ends is their way of showing off their Chessmaster cred, or maybe they're just Wicked Cultured. The downside of this, of course, is that any hero can bring the plan crashing down simply by disrupting the song.
The Xylophone Gag is a very specific subtrope of this. If the song itself is harmful, it's a Brown Note. If the song is part of a magical spell, it's Magic Music. If the song opens a door, it's an example of Songs in the Key of Lock. May involve a Cacophony Cover Up.
- In Read or Die, a clone of Beethoven is a big part of the I-Jin's plans; his Suicide Symphony causes anyone who listens to it to commit suicide.
- Elfen Lied has a music box playing the theme song "Lillium" that causes Nyu to shift to her virulently murderous side Lucy.
- Sorta used in Detective School Q, where the Banquet of Evil melody is used to bring a violinist insane with guilt and fear - years after he and other violin players conspired to get their biggest rival to inherit their teacher's prized violin out, which results in her being mutilated and commiting suicide afterwards.
- In Olga Chigirinskaya's novel Beyond the Dawn, a particular song performed by a particular minstrel was the key to undoing the protagonist's Memory Gambit.
- In Memento Vivere, a Final Fantasy X fanfiction, most appearances of the Songstress Dressphere invoke this trope, as it creates Magic Music.
- Oblivion (Gabriel Seraph) has one where by playing the intro to "Madness," Tim can change his guitar into any stringed instrument.
- The Man Who Knew Too Much: One specific cymbal clash in the symphony was the assassin's cue to fire—that was the loudest moment of the symphony, so it would have masked the sound of the gunshot.
- Zoolander: The song "Relax" causes Derek's brainwashing to kick in, transforming him into a mindless assassin. This is due to the fact that Big Bad Mugatu was The Pete Best of Frankie Goes to Hollywood.
- Get Smart: The final notes of Beethoven's 9th Symphony are the signal for the detonator.
- Eagle Eye: The pentultimate trumpet note of "The Star-Spangled Banner" is the signal for the detonator.
- Also an example of poor research, as the same note is played much earlier... "And the rocket's red glare." But then, the bad guys specifically arranged for that last note to be extra-long.
- Serenity: River Tam has been programmed to go on a killing spree when a subliminal signal, hidden in a Fruity-Oaty Bar commercial, is played.
- In The Living Daylights, James Bond has a keyring that emits a stunning gas when he whistles Rule Britannia and explodes when he gives a wolf whistle (although Bond jokingly asks if the explosive was set to "God Save The Queen").
- In The Three Stooges movie Punch Drunks, Curly goes berserk whenever he hears "Pop Goes the Weasel".
- Likewise, in the Laurel and Hardy movie Saps at Sea, Oliver Hardy goes berserk whenever he hears horns.
- In Inception, the team uses "Non, Je Ne Regrette Rien" by Édith Piaf to time their awakening from the lower dream levels. At the end of the song, their accomplice on the first level will crash the van they are all in into the river. They need the musical countdown due to the time dilation effect.
- Mission: Impossible – Rogue Nation pays homage to The Man Who Knew Too Much example above. At a performance of Turandot, Ilsa uses a note from "Nessun Dorma" as a cue for her shooting the Austrian chancellor.
- In Willy Wonka & the Chocolate Factory Wonka opens the doors to the Chocolate Room by playing a fast sequence (misidentified by Mrs. Teavee as "Rachmaninoff"; it's actually from Mozart's The Marriage of Figaro) on a keyboard.
- In A Clockwork Orange, Beethoven's 9th Symphony induces severe nausea in the film's sociopathic protagonist, Alex Delarge.
- In Family Matters, one episode had Steve Urkel invent gunpowder that was set off by the finale of "Roll Out the Barrel." When it was mistaken for pepper and used to cook dinner for a party, hilarity ensued.
- Buffy the Vampire Slayer: Spike's trigger in season 7 is an old English folk song, "Early One Morning."
- In The West Wing, Josh is counseled by a psychologist due to his recent unpredictable behavior, and it turns out the trigger was the Christmas music in the lobby. In his mind, music represented the ambulance siren and made him relive the trauma of being shot months before.
- An episode of Get Smart featured a cannon that would fire at the climax of Tchaikovsky's 1812 overture.
- In another episode, titled "Weekend Vampire," this was also the secret behind the mysterious vampire killer; he used a special modified oboe to fire two poisoned needles into the victim's neck when he played high C.
- Battlestar Galactica - the jump co-ordinates needed in the series finale were derived from Starbuck playing the main theme from "All Along The Watchtower". While being nuked near a black hole, no less.
- In Game of Thrones, the playing of "The Rains of Castamere" at the Red Wedding is the cue to lock the doors and murder everyone inside that isn't a Bolton or Frey.
- In the penultimate episode of Community's third season, Chang has rigged the documents incriminating him in his brutal takeover of the school to be set on fire when he hits the high note in his awesome keytar solo. Unfortunately, his plan will also result in the building burning down, because despite his claims that fire is not a ghost, it can in fact go through doors.
- Season 1 of The Sinner starts with Cora Tanetti suddenly murdering a man on the beach when a song that his girlfriend played from the man's college band is played. Another episode has Detective Harry Ambrose play the same music to her, making her attack him in the same exact positions she stabbed the man to death. It's because that song was playing when, during a drug-fueled orgy she took her sick little sister to, that song was playing and the sister died in the middle of it and he ended up crushing her chest trying to give her CPR and she attacked believing he killed her.
- 7 Yüz: In "Hayatın Musikisi", Oşa conditions Pınar to trigger her "confidence mode" by singing or hearing Ajda Pekkan's "Bambaşka Biri". The title of the song — the Turkish version of Gloria Gaynor's "I Will Survive" — literally translates to "Someone Different", signifying the personality transformation she undergoes after induction.
- The most famous example is the eponymous instrument from The Legend of Zelda: Ocarina of Time - Zelda's Lullaby - is basically a generic activator for a wide variety of effects. Brawl in the Family took the Lullaby's relationship with the Triforce to its semi-logical conclusion.
- The Forgotten Superweapon Vegnagun in Final Fantasy X-2 is operated by playing an elaborate tune. Good thing the only person who remembers it is dead... Wait, he's come back as an Unsent? Oh, Crap!.
- Left 4 Dead 2: Get Jonathan Coulton's Re: Your Brains song to play on the jukebox and the chorus will result in a Horde being summoned.
- This is how you get the best ending of Chrono Cross. In the fight against the Time Devourer, each element color plays a corresponding note when used. You free Schala by using them in the right order to play a specific tune.
- Shivers (1995). By playing the siren's song on the organ in the "Mysteries of the Deep" room, a door opens up to reveal the way to the "Subterranean World" maze.
- Psycho Mantis from Metal Gear Solid uses "mind control music" to control Meryl. When you hear it on the way to the commander room, looking in first person view shows the perspective of Mantis looking at you implying that you are also being manipulated.
- Inverted in BioShock. Sander Cohen ties a former student of his to a piano, which is rigged to explode unless the performer successfully plays a specific song composed by Cohen. Unfortunately, "Cohen's Masterpiece" is a song specifically designed to be nigh-impossible for a pianist to complete, and missing too many notes causes the bomb to go off. It goes off.
- This forms the backbone for the EXA_PICO franchise where songs are used to relay emotions and get all kinds of effects.
- Extremely subtle, but most Dark Souls bosses are this. Each one has it's own theme, and the bosses move to the rhythm of their theme. It's also no the soundtrack, but ambient music (which means the character must be hearing it: often because the boss themself is making the sound). This is part of the games series' "tough but fair" design, because the player eventually starts to feel the boss's rhythm. Most obvious with the Dancer of the Boreal Valley, because her theme music is very noticeable and (being an optional boss) she's even more challenging than usual.
- In Dreamfall: The Longest Journey the only way for Zoë to get out of the cave under Marcuria is to play a certain tune in order to open a secret door. So how can you guess the tune? It is hummed by the grubbers you encounter at the beginning of the level. If you kill them, you have only one more chance to hear it, which happens before you even know that there is a secret door. The same secret door comes up in the sequel too, but here the game is more merciful. Crow sings the tune to Kian every time Kian fails to replicate it properly.
- To open Sammy’s sanctuary in Bendy and the Ink Machine you have to play instruments in a certain order.
- In Final Fantasy VII, once you have the Highwind, you can go back to Tifa's house in Nibelheim, locate the piano and play the first few bars of the main theme. Once you play the right notes, you acquire Tifa's ultimate Limit Break, "Final Heaven".
- In Girl Genius, Agatha uses an organ to control her "battle circus".
- Looney Tunes and Merrie Melodies: A standard "explosion" gag, where the cartoon's antagonist — trying to blow up his foe — attempts to attach dynamite or other explosives to a certain key, hoping that his unwary opponent will press it while playing the song, always "Those Endearing Young Charms." However, the good guy keeps up his guard and (perhaps intentionally) plays the incorrect note to avoid pressing the wrong key; after a few attempts, the frustrated antagonist will shoo off his opponent and demonstrate the correct playing of the song ... always leading to playing the rigged note and taking the explosion.
- Classic-era Warner Bros. appearances include "Ballot Box Bunny," "Show Biz Bugs" and "Rushing Roulette."
- The Looney Tunes use was Lampshaded and played with in Animaniacs, where Slappy Squirrel is presented with an obviously rigged xylophone. Both she and her nephew Skippy are Genre Savvy to the old cartoon trope, but against Skippy's protests Slappy plays "Those Endearing Young Charms" anyway — and she somehow makes the antagonist explode twenty feet away when she hits the trigger keys. It then becomes a Brick Joke when the antagonist later crashes into the xylophone and falling nuts start hitting the keys...
- South Park: Don't forget the late Middle Park Cowboys' mascot, to whom was attached a bomb rigged to explode when John Stamos's brother Richard hit the high F in Lovin' You. Of course, everybody knows Richard Stamos can never hit the high F... Until he finally does in the last few seconds of the episode.
- A Pinky and the Brain cartoon involved Brain encoding a subliminal message in a typical country song.
- The Schmëerskåhøvên song makes everyone dumber and dumber. Probably counts too.
- Chaotic uses Mugic, musical magic.
- In an episode of TaleSpin, the gate to a lost Advanced Ancient Acropolis is opened by singing a perfect B note, which the first goon sadly cannot.
Klang: She said B, not B flat!
- Thunderbirds offers the cham-cham, a coded musical instruction to air pirates signalling the time to attack and directing them to their target. When this is discovered, Lady Penelope sings an altered version of the song, redirecting the hapless pirates' flight-path right over a military airbase.
- In the Batman: The Animated Series episode "If You're So Smart, Why Aren't You Rich?", a door has three possible keys, labelled "A, C, and D". The D key causes two blades to fly at Batman and Robin. When Batman tries to use the A key, Robin stops him, saying there will be three blades—the Key of A and the Key of D have three and two sharps, respectively. The correct answer is the Key of C, which has no sharps. They get through the puzzle.