Re: the music thing, I think in principle all you'd have to do is invert the music track and then have the app sync that sound file with the background music such that the sound waves would cancel out. Essentially you'd just have each area's track recorded invertedly, which you'd buy for the app, and then when you walked into an area, the program would have to detect where the background music was at in its "track" and sync up the inverted music file with it.

(also if my science fails me here, it's 2 AM, don't judge me)

The other logistics of how to play it, what range it would have, etc. would be something you'd have to work out separately, but in terms of just flipping the tracks, I could always throw you Carmen's Porygon. He does sound editing sometimes. It's a hobby. >>