Run a Metro app, and you'll notice one thing is missing --- there's no apparent way to close it down. Usually, this isn't a problem, just let it run in the background. But there are times when an open Metro app may drive you off the deep end. For example the Zero Gravity app has a soundtrack designed to set your teeth on edge. Switch out of the app, and the soundtrack still plays.
         There's a simple if kludgy workaround. Get back to the main Metro interface, then click the Desktop tile. That will switch you to the Desktop, and the music stops immediately. The app will still run for a minute or so, but the sound will be off. But after a short while, the app itself will shut down.