Nothing that I know of, other than I shrank & enlarged the resolution as Boyd suggested. That didn't work immediately but when I rebooted next day, it was all good again. Or maybe it happened when my dog stood on the connection to the amp on the PC's sound & I jiggled things around to get the cable out to resolder.
Actually, I'm betting that it was something to do with Picasa. Every time my wife posts a web album, Picasa screws up one or other of my settings.