USB Audio does not work on anything but WMP?

My computer is hooked up to a USB stereo system. For the first 6 months I've had this computer, it has worked without any problem. Recently, however, the USB system works only with Windows Media Player. I've been forced to use the poor speakers on my display for everything else but the music on WMP. This has been going on for a couple months now, and I'd like to know what's up. Anyone have a clue?