Essays
Sound arrives first
Ask people what made a scene frightening and they will describe what they saw. Take the sound away and the scene is usually not frightening at all.
The reason is timing. Sound reaches an audience marginally before they have finished reading the image, so it sets the terms on which the image is understood. A door opening onto silence and a door opening onto a low room tone are the same shot and two different scenes.
The room is a character
The most consequential decision in a mix is usually the quietest one: how much of the space to keep. Dialogue can be cleaned until it sounds recorded in a vacuum, and many films do exactly that, then spend the rest of the mix trying to put the room back with reverb. Films that keep the original room tone feel inhabited, and audiences describe them as realistic without knowing why.
A cut is invisible if the sound crosses it. A cut is brutal if the sound stops with the picture. Neither choice is neutral.
Foley and the lie of accuracy
Foley is not reproduction. Real footsteps recorded on location are thin and unconvincing; the ones in the film were performed later, by a specialist, in shoes chosen for character rather than accuracy. A heavy man is given a heavier step than he actually has, because the audience is listening for weight and not for truth.
This is the part of film craft with the widest gap between its importance and its visibility. It is also the part most damaged by compressed playback, which is how most people now hear films.