Your system was already set for sound, but NOT USB sound it seems.
i have win 7 ulti. i just got some wireless plantronics 995 headphones for xmas. i plugged em in and now on youtube the videos will not play at all while the usb is plugged in, remove plug video starts up, plug back in it stops again. also i tried setting headphones as default in system to no avail. i got 5 seconds of sound from them when i first plugged them in , now i cant even get sound when i "test" them in sound config