Nice documentary short talks about the history of sound in film and the new techniques being used today (or 1950). This is a pretty good short as it does a nice job at showing exactly what a sound mixer and designer do.

http://www.imdb.com/title/tt0240015/

We get some clips of early silent movies and the advances that Edison tries to make in order to get sound on film. We then flash forward to LIGHTS OF NEW YORK and IN OLD ARIZONA, which I just watched a few weeks ago. The short even points out a few of the scenes where the microphone was hidden. We also get to go inside a sound vault to see where they keep all the sound effects and how they are labeled and stored.