As far as I recall from one of Sergey's posts on that, the kernel isn't configured for USB audio in the image, so you'd need to reconfigure and rebuild it. You can look at his instructions for some insight, however the changes he describes there were not specifically related to USB audio, so it may be not that useful.
Let's see if "@-mentions" work here :-)
SergeyK - care to share some more specific instructions on enabling USB audio?