Here's a better version of this technology for movie theaters, and I'll give it away free: Movie manufacturers can add a short ultrasonic code at the beginning of each film's audio track that alerts the phone a movie is starting, and also encodes the film's length so the phone knows when to turn back on. Phones can run a free "Movie Silence" app that runs in the background (or better yet, bundle it into the OS to save on overhead).
Of course, for safety and user control the "Movie Auto-Silence" option should be a toggleable setting with emergency overrides, probably for specific contacts or in certain locations. (i.e. you wouldn't want your phone silencing itself at your own house)