Sound technique research

What is sound in film?

Sound in film is used in balance to create necessary emphasis on certain aspects, which in turn create desired effects for the audience

Diegetic sound

This is sound that can be heard by the characters in the scene and refers to all those audio elements that come from sources inside the world we see on the screen, including dialogue, doors slamming, footsteps, etc.

An example for this are the noises that can be heard in Batman: Dark Knight in the interrogation of the Joker:

Non-diegetic sound

This is sound that the characters cannot hear and it is not part of the imaginary world of the story. This includes musical soundtrack or a voice over, however this exclude narration by a character within the story - referred to as an internal monologue and is classed as diegetic. It includes sound effects.
An example of this is in the Hobbit:

Music/score

The music used in a film is normally specifically composed to accompany the film. It includes sound effects and dialogue and forms part of the films soundtrack. It comprises of a number of orchestral, instrumental or choral pieces called cues which are timed to begin and end at specific parts in the film. Music helps to enhance the emotions felt in the film and create more drama within the narrative. Background music and diegetic music are the types of music used in films, and it aims to create and atmosphere, and to move the action forward in the narrative.
An example of this is in Skyfall, where a song was composed for the film:

Sound motif

This is a sound effect or combination of sound effects that are associated with a particular character, setting, situation or idea.

An example of this is in Fast and Furious:

Sound bridge

A sound bridge can lead in and out of a scene. They can occur at the beginning of one scene when the sound from the previous scene carries over briefly before the sound from the new scene begins. It stresses the continuity form one scene to another as the mood in the film remains the same. 

An example of this is in The Matrix:

Ambient sound 

These are the background sounds which are present in a scene or location. Common ambient sounds include wind, water, birds crowds etc. It performs a number of function including: providing auddio continuity between shots, preventing an unnatural silence when no other sound is present and establishing or reinforcing the mood.

An example of ambient sound is Angus Thongs and Perfect Snogging at 3:50:

Voice over

When a voice, often that of a character in the film, is heard while we see an image of space and time in which that character is not actually speaker. The voiceover is often used to give a sense of a characters subjectivity or to narrate and event told in flashback. 

An example of this is in The Great Gatsby: