Sound

Definition:

Literally everything that can be heard on the film. Sound for film is made up of three layers; dialogue, sound effects and music.

Uses:

Sound shapes the viewer's interpretation of the visual images through engagement of a second sensory pathway.

Sound enhances emotional responses and can signify plot developments, character psychology or reaction to events.

Sound is never benign in the world of film, even the absence of sound has a purpose.

Diegetic sound: Any sound that is part of the world of the narrative.
Non-diegetic sound: Any sound that is external to the world of the narrative.
For example, the piano music in Shine is diegetic or within the narrative when Helfgott is playing the piano, but is non-diegetic or external to the narrative when mood music is used.