What USB sound card have you got? I have the Startech Virtual 7.1 USB Audio Adapter (model No. ICUSBAUD107) and it is plug'n'play - if it isn't, then my understanding is that you need to install it first as you would with any Linux system, and then go from there. It might even be appropriate to make a template for it.
To get you started, you can check if it is being recognised with "lsusb", and then Google accordingly.
No idea how to stop it from defaulting to the internal sound card - blacklist the module for it perhaps? <-- Complete guess, proceed at your own risk.
We're using those successfully on some of our systems and they work very nicely indeed. In fact I'm just prepping one of our compact NC25's now with 2x Startech Virtual 7.1 USB Audio Adapters. Its streaming 8x Internet Radio stations on our test rig right now.
All the best