Just so you know, USB speakers don't use the sound card built into your PC. They have their own soundcard built into them. For them to work the correct drivers
for that device must be installed. The data is sent over the USB bus to the sound card in the speakers. Now having said that, if these are just USB Powered
speakers and they have an actual analog audio cable that plugs into the speaker jack. They are driven from the onboard sound card just like any other set of speakers. In that case you want to make sure they are plugged into the correct audio jack and are getting enough power from the USB bus.
EDIT: OK it looks like they are USB powered speakers and no driver
is required. Other than the one for your sound card that is. It looks like its a sound card or jack/detection problem.
SECOND EDIT: I read everything over again and it sounds like you need to install the driver for your onboard sound card. Oh and please confirm for me that they are USB Powered speakers and have two cables. A USB cable and an audio cable.