Here is a link to a website devoted to an often forgotten aspect of the very visual film medium: Sound. Virtually everything you hear in a modern film from dialog to background speech (walla) to foley (mostly footsteps) is either artificially added or post processed in a hundred specialized ways. It’s a testament to the skill of these artists that films sound as good as they do.
It wasn’t always that way, one of the reasons people find early sound films a challenge is the primitive nature of the sound. Often the only sound system was an open omnidirectional mic. This was also a few years before high quality condensor mic systems escaped from the Navy sonar department and into studios.
filmsound.org is a great resource into this niche but fascinating field of film. From basic how to articles to extensive and detailed interviews, this site is a treasure for any film fan who wants to see how their favorite art form is made.