this happens to me sometimes because i use a pair of earphones and a headset mic that i hang around my neck, and for some reason it'll send the sound through the headset that is muted. All I do is unplug both of them, and then plug the earphones in and then my headset and that fixes it.

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I have a fix for this problem, give me a few minutes to find my old thread for it. Not sure where I posted it.

Edit: I'm assuming you have the ASUS Xonar software?
Let me know if the solution in post 2 works for you
http://teamfortress.tv/forum/thread/12260-sound-card-streaming-issue