Little Cocoa app that monitors the room volume via a computers microphone and reduces the system sound when the room volume goes up. The net effect is that if you start talking to somebody, the music goes down in volume. When you stop, the music goes up again.

This is almost certainly too simple and just wouldn’t work because of pollution by the music that’s playing. But you can tap into the audio output on Mac OS X, so you could filter everything coming from the computer out by comparing the input to the output, accounting for a certain amount of distortion.