• Aid to filmmaker
• Aid to audience
• Manipulation of the audience
• Creation of a subjective environment
• Creation of a sympathetic environment

Here are some quotes from the Walter Murch playbook:

“It (sound) should strive to create a purposeful and fruitful tension between what is on the screen and what is kindled in the mind of the audience. The danger of present- day cinema is that it can suffocate its subjects by its very ability to represent them: it doesn’t possess the built-in escape valves of ambiguity that painting, music, literature, radio drama and black-and-white silent film automatically have simply by virtue of their sensory incompleteness — an incompleteness that engages the imagination of the viewer as compensation for what is only evoked by the artist.” -Walter Murch

“By comparison, film seems to be “all there” (it isn’t, but it seems to be), and thus the responsibility of filmmakers is to find ways within that completeness to refrain from achieving it. To that end, the metaphoric use of sound is one of the most fruitful, flexible and inexpensive means: by choosing carefully what to eliminate, and then adding back sounds that seem at first hearing to be somewhat at odds with the accompanying image, the filmmaker can open up a perceptual vacuum into which the mind of the audience must inevitably rush.” -Walter Murch