Mmmh... looks good. I don't know how hard it could be to program and implement it. But I guess it could work. And it should be quite easier than changing every application.

But there should, in my opinion, be a fast way to silent a event on a fast way. Think about you're sitting in a conference or in a cinema. But it could simply work only by tapping the screen with a uncovered proximity sensor. It has just to silent every sound output until the event disappears or the screen is unlocked by the user. (slider on the screen or the unlock switch at the device)

Please add it as a solution to the Brainstorm. I will vote for it!
If you don't know how, please feel free to ask.