I suspect that your PC has got confused by the various drivers. It might have been better if you had disabled the on-board sound card before installing the USB one. And make sure you don't install the sound drivers from you MB installation disc. Also some devices create problems if you u do things the wrong way around - some expect you to install the drivers before you plug in the device while others insist you do it the other way round - you will need to check your installation guide to check the correct process. Whether you can repair rather than start again is in the lap of the gods. You can try inserting the sound card into another USB port, but first I suggest boot into safe made and you ensure all the sound drivers have been uninstalled.
iMac Pro review