You may have noticed that in every audiovisual work, characters seem to have music playing in their heads at all times. It's not music playing on a radio in their car and they aren't at a concert. It's not songs they've heard. Not even classical tunes they’ve listened to. These are tunes that seem to just come out of nowhere, off the top of the character's head.
This technique is used because it makes you sympathize with a character's emotions. Without this music, you might not be able to connect with characters and feel their emotions quite as easily.
When music is used in movies and TV shows, there's two types: in-universe music the characters hear and music that the in-universe characters don't hear, because it's for the audience only. The first type is Source Music, aka "diegetic music", which is music you hear that comes from an in-universe source, such as the character's car radio as they're driving, a band at a bar, a street musician outside, or an instrument the character is playing.
If music is shown in one character's perspective, but then the perspective switches to another character who can't hear the music, that's Sudden Soundtrack Stop.
This trope lists lampshades of the phenomenon, such as when characters hear the background music, often Played for Laughs; straight examples are too many to count. For subversions, see Left the Background Music On. For aversions, see Reality Has No Soundtrack.
Compare Mickey Mousing, for when music that is meant to set the mood is actually being played by a character in-universe.
- The Far Side has several gags where characters can hear the background music:
- One featured a shot of Tonto knocking on an outhouse door, whispering, "Kemosabe, hurry up, the music's starting!"
- A group of cowboys at camp looking around: "There it goes again — whenever we settle in for the night, that harmonica starts up!"
- A gal standing in the aftermath of a huge bar brawl recounts it to the police: "So this little sailor dude whips out a can of spinach, this crazy music starts playin', and... well, just look at the place!"