Sound Glossary

Diegetic sound – sound that comes from a person or object in the diegesis (the world of the story) and seen within the field of vision.

Non-diegetic sound – sound that comes from nothing within the field of vision and has been added afterwards in the editing process. The clearest examples of this are soundtrack music or voiceover.

Soundtrack – a recording of the musical accompaniments to a movie.

Theme – The subject of a talk, a piece of writing, a person's thoughts, or an exhibition; a topic

Tune – A melody, esp. one that characterizes a certain piece of music

Incidental music – Music used in a film or play as a background to create or enhance a particular atmosphere

Sound effects (FX) – A sound other than speech or music made artificially for use in a play, movie, or other broadcast production

Ambient sound – In atmospheric sounding and noise pollution, ambient noise level (sometimes called background noise level, reference sound level, or room noise level) is the sound pressure level at a given location, normally specified as a reference level to study a new intrusive sound source.

Dialogue – Conversation between two or more people as a feature of a book, play, or movie

Voiceover - Voice-over (also known as off-camera or off-stage commentary) is a production technique where a voice that is not part of the narrative (non-diegetic) is used in a radio, television production, filmmaking, theatre, or other presentations.

Sources: http://www.slideshare.net/raybloggs/glossary-of-sound-terms-for-tv-drama and Google web definitions