My primary audio card on my desktop PC is an M-Audio Fast Track Pro and after I had upgraded to the retail version of Windows 7, it would periodically just stop working (dropout).  The sound would not play and some videos would refuse to play since no audio output was available. This would usually happen after I had been away from my computer for a few hours, meaning after no sound had been played for a while. I suspected it might be shutting off after inactivity, but it would also happen only a couple minutes after the most recent audio was played.

I did the usual and made sure I had the latest drivers, uninstalled, reinstalled the device, changed USB ports, etc. I also followed some advice on the M-Audio forums including checking for IRQ conflicts. But the problem persisted, and I finally went to check what I had suspected all along: Windows’ USB power management

THE FIX: Go to Device Manager > Universal Serial Bus controllers > and then right click Properties on all instances of USB Root Hub, and in the Power Management tab, make sure the first option is unchecked. (Source: M-Audio FAQ)