Sound Terminology

Sound:

Everything we hear from the audio track of the film.

Music:

Any music that comes from the audio track.  Music might be diegetic (a song on the radio of a car a character is driving) or non-diegetic (scary music when a villain appears on screen).

Diegetic Sound:

Sound that the other characters would be able to hear, a song on a radio, for instance, as a character drives down the highway, would be a diegetic sound, as would someone coughing audibly during a scene.  It is important to note that diegetic sound is a sound that characters could hear, even if they are not present when that sound occurs.  The sound of a radio playing in an apartment, for instance, is a diegetic sound, even if no character is present in the apartment during the scene.

Non-diegetic sound:

Sound that characters cannot hear, the two most common types of non-diegetic sound are voiceovers, which is a character’s narration that plays over any given scene, and non-diegetic music, which is music used to inflect the mood of a given scene.  Creepy horror-movie music, for instance, that plays when a character is walking into an old house, is non-diegetic music, since that character cannot hear the music.  Sometimes, this effect is parodied (with characters commenting on the scary music playing), and some directors will transition from a non-diegetic sound to a diegetic sound (or vice-versa), as when a song is playing on a radio that then becomes the non-diegetic music even as the characters move into a new scene without the radio in it.

Ambient sound:

This term generally refers to any sounds that are used to establish location.  The ambient sound of a scene in a park, for instance, might include birds chirping, children laughing, or a dog barking.  The ambient sound of a train station would include the whine of train brakes, the tinny sounds of arrival and departure announcements, and the general noise of people walking and talking.