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The use of audio, whether it is dialogue, SFX or scored music, is just as important to a film as the images that are produced on screen.
Some of the most memorable moments in cinematic history are remembered as much for the aural content as the visual content. Two of the most obvious examples of this are Steven Spielberg's 'Jaws' and Alfred Hitchcock's 'Psycho'. I would guess that most people reading this are now hearing those famous sounds in their head. It is a testament to the people who worked on the audio for these pictures that so many people can recall these pieces of music.
Audio has many uses when used with moving images. The most obvious are dialogue and sound effects that reinforce physical actions that are taking place on the screen (sound of people walking, doors creaking, cars driving past etc) This is the audio you will hear the most in the majority of films. Most of this audio is 'expected' and is required to make the scene sound realistic. However, in instances such as the shower scene in 'Psycho', the actual sound of the stabbing is replaced by instruments approximating the stabbing sound in a high musical register.
Music is often used to give, or strengthen, a sense of location and or/time. If a film was being made about a group of teenagers from Seattle in the early 1990s it would quite logical to use some music similar to that by Nirvana, Pearl Jam, Soundgarden, whether as incidental/background music (such as on a jukebox or on the radio) or as a more 'up front' part of the soundtrack. Cameron Crowe's 'Singles' does exactly this.
Conversely, a film set in mediaeval France would not be well suited to heavy rock music and would be better suited to music in a sympathetic style to the location and period, such as music from the 'ars nova'.
This use of audio to help set time and location relies heavily on cliche and you will notice this in many films - samba drums for Brazil, bagpipes for the Scottish etc
Music can also be used as a 'glue' to hold together many scenes that may otherwise have appeared disjointed. The most obvious example of this is a montage sequence, as famously parodied in Trey Parkers 'Team America: World Police'. This can aid the passage of time and therefore move the narrative further ahead and can remove the need for dialogue.
Audio is also important in creating an emotion in the viewer and this can be performed in many ways.
One of the most common emotions that audio is used to reinforce is that of suspense/impending terror. If you watch a suspenseful scene in a horror movie with the sound muted, much of the suspense is removed. The audio in scenes such as this are integral to the emotion being constructed in the viewer.
The feeling of suspense is usually created by appropriate music scoring/instrumentation along SFX. The SFX in a scene like this can range from the sound of water dripping and subway trains rumbling in the distance if the scene is set in New York sewers, to the sound of the main character breathing and his/her heartbeats - this is more common in POV (point of view) shots.
Another powerful use of audio is when the audio 'contradicts' the visuals. This is more often music although there are films where the SFX do not follow the visual action. This could be a 'beautiful' piece of music over a 'ugly' scene - the climactic scene involving Leyland and Laura Palmer in David Lynch's 'Fire Walk With Me' uses this device to invoke a different reaction in the viewer than if the music had been more typical for a murder scene.
One final interesting use of music in film can be found in Paul Thomas Anderson's 'Magnolia'. Near the end of the film as most of the characters reach the apex of their individual emotional arcs, the Aimee Mann song 'Wise Up' is played in the foreground. Throughout this song, the main characters in the film all sing along although they are all in disparate locations and in varying states of emotional and physical distress. This unusual approach added some dark humor to the already taut emotions that the viewer was feeling whilst the songs words further enforced the emotional trials that the characters were going through.
