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A character is left alone, and the soundtrack falls silent. Completely silent... that is, unless you count the dripping faucet in the corner. Oh, and there's that chirping cricket on the windowsill. Then The Thing That Goes Doink comes in at measure 12...
In short, a sequence where natural background ambient sounds are woven together into a coherent musical piece. Can sometimes be used as nature's equivalent of the Unguided Lab Tour.
This trope can be usually seen in particularly creative advertisements. In Real Life, this kind of music is called an Ostinato, though it doesn't HAVE to be just background noises; any repetition can do ("The Mysterious Ticking Noise", for example, also is an Ostinato).
See also Everything Is an Instrument. May result in a Trash Can Band.
KitKat commercials do this with the sounds of opening, eating, and humming enjoyment of the candy bar to the "Give me a break" theme.
One of Nike's most famous ads involves a bunch of NBA players creating a Serendipitous Symphony with their shoes, the ball, and the net. In fact, it was so famous, it was even parodied in Scary Movie 2.
Another ad, for Levi's, featured sounds in an ER gradually forming "Tainted Love," with the patient and eventually the entire staff singing along.
A classic ad for Maxwell House Coffee involved a percolator brewing, and as it heated up, the pumping noise it made became the Maxwell House jingle. The jingle continued to be used well into the 90's, several decades after percolators had mostly gone out in favor of automatic drip coffee makers.
In Little House on Rocky Ridge, as Laura, Almanzo and Rose pass through some city or another on their way to their new home, they get stuck in traffic with a herd of cows with bells around their necks. Rose notices that the bells play "Ta-Ra-Ra-Boom-De-Ay," which she had gotten her parents to sing over and over again when they first set out.
The Song of the Cloud Forest from The Jim Henson Hour features one a little bit after the opening.
Tyres from the Brit ComSpaced is a such a raver, he hears random enviromental noises as rave music, resulting at one point to him dancing to the beeping of a pedestrian crossing. On another occasion, he starts building up a Serendipitous Symphony, but Tim deliberately gets rid of the noises to stop him so he'll get on with making an urgent delivery.
Nickelodeon used to run a series of shorts called The Space Between Mr. Frear's Ears, which consisted entirely of this trope.
These segments were done by Stomp. And is arguably their entire act. (Suppose that makes this Truth in Television then.)
Roseanne had an end credits segment where the guys were sitting around in the kitchen, making random noises... Which they turned into a melody... And then busted out into a full-on rendition of The Beverly Hillbillies theme song. This was probably done more for the enjoyment of the audience, rather than meant to be in-character, though (and that's not how it happened at all anyway).
Flight of the Conchords does something like this on the song "Leggy Blonde". While it's an actual song with guitars and shit, the rhythm section is made up of various office supplies (staplers, printers, tape etc.).
In Scrubs, JD once stalled for time while thinking of something special to do for Dr. Cox. As he tapped his pencil, thinking, the events around him converged into a a serendipitous symphony, which Cox assumed JD had planned out.
In another episode, JD came to work wearing headphones and musing on how things seemed to go together with his music, including Doug trying to revive someone he'd killed, nurses handing clipboards back and forth, and the janitor sweeping, up to Dr. Kelso pulling his headphones off while asking the question that was the last words of the song, "Are you having a good time?" and yelling at him some for daydreaming. As JD walks away, the Janitor is singing the same song to himself.
Wings has a moment where everybody is bored because the airport is fogged in. Antonio is clinking his spoon in a coffee mug, Joe is noodling on a guitar, and Brian is flipping through a magazine. Then Lowell starts sanding the door frame nearby. His rhythmic sanding coincides with Antonio's clinking beat. Suddenly Brian starts scatting, and they're doing a full-on Bossanova number. Eventually they realize what's happened and pause——before continuing right on with the music.
Happened in a 10th-season episode of Top Gear, when the crew start comparing the various noises given off by their British Leyland cars. The various squeaks, rattles and clunks soon turn into a musical number...
Sponge Bob Square Pants did this when luring party animal jellyfish out of his house. It starts with Gary's eyestalks clacking together, and builds up from there.
In the Phineas and Ferb episode "Dude, We're Getting The Band Back Together!", one of these in a library, of all places, kicks off the number "Ain't Got Rhythm".
Our heroes try to start one of these while cleaning a bathroom with toothbrushes in "Phineas and Ferb Get Busted", but then the Drill Sergeant Nasty yells "And no funky rhythms!"
In the Simpsons episode "Hurricane Neddy", a hurricane blows into a harmonica shop, creating a pleasing melody. The same thing then occurs with an adjacent harpsichord shop, resulting only in a discordant thunk.
In "Homey The Clown", Homer rides his unicycle along a bar, with wine glasses striking his head. The notes produced by these glasses form the opening bars of the Godfather theme.
In the Popeye cartoon "Me Musical Nephews", Popeye's nephews are bored until they decide to make music with various objects in their room.
My Little Pony: Friendship Is Magic has started off two musical numbers like this: "The Super Speedy Cider Squeezy 6000" with the sounds of the eponymous machine; and "Apples to the Core" with rattling pans and bumps in the road, in a clear reference to "On the Open Road" above.
Industrial processes tend to be quite prone to this, as the cyclical operations of a machine are inherently rhythmic in nature.
The A Cappella group, Toxic Audio, does a cover of "Put da Lime in da Coconut." It starts with someone sporadically coughing. Then another member starts clearing their throat. The soprano starts sneezing in time. And suddenly the whole range of ailments becomes the background beat for "Coconut."