Audio.

Background music.
 + Emotion
 + Rhythm
Usually not meant to be noticeable.
Provides a tone or emotional attitude toward the story/character.
Foreshadow a change in mood.
Aid viewer understanding by linking scenes.

Dialogue.
 - Authenticates the speaker as an individual or a real person.
 - Tell the story/express feelings/motivations of characters.

Diegetic/actual sound.
Non-diegetic/overlayed sound.

Synchronous sounds.
Matched with what is viewed - see what is making the noise.
Makes it more realistic.
Different volumes in order to direct the audience to certain sounds.

Asynchronous sounds.
Not matched with the viewed source.