I have exactly the same issue, it's really annoying, say you pause a video you're watching, then u press play all sound is gone, so you have to go to your sound device and uncheck some options, press apply, then re-check the options, and sound comes back... I also notice I cant play audio CDs in WMP11 in Vista... So I think it's driver issues, and I've been scouring the web for a long time to try and resolve this with no luck yet.
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