Go into your device manager (rightclick on My Computer, Properties>Hardware tab>Device Manger) and find your USB Sound Card. (probally in sound, video, and game controllers) From there, double click on your card and there should be a Power Managment tab, if so, make sure nothing about "saving power" or "Let Windows Turn Off Device To Save Power" is checked.
Thats my guess, but I could be very wrong... There may not even be a Power Management tab in there... But, its worth a shot