diegetic music is common in movies. the characters in a film hear something and the audience hears the same thing. if a character in the film turns on a cd player and listens to music and the audience hears it, this is called diegetic. in the film "titanic", a string quartet plays music while the ship is sinking. the people on the ship hear the music and the audience hears the music. in "apocalypse now", helicopters are raiding a village. the leading colonel turns on a loud stereo with the music the ride of the valkyries by richard wagner. the soldiers hear this music and the audience hear it too. when marty mcfly plays his guitar in "back to the future", both the characters in the film and the viewers hear it. marty plays music from the future. the listeners in the film do not know the music, but the music is still diegetic.