I've had occasional audio playback problems. My symptoms are that websites and videos will suddenly stop making any sound in firefox while I can still play tunes in vlc. Restarting firefox makes no difference. If I fire up other applications, they stop making sound as well.
It turns out that there's a problem with the version 5076 drivers for the M-Audio Delta 1010 and 1010LT on 64 bit Windows 7. It can be solved by restarting the windows sound service.