Hmmm... I run my onboard audio in tandem as well, which is not preferred, but works as long as my ASIO device is not "default" when on.
Is there anything listed in SONAR that you can choose from when you select ASIO mode? I just shifted to a new interface and the old one was embedded into Win7, so ASIO in SONAR would only see that one... I had to unplug the old one from the system.
I would check a few other things...
1) Check drivers for updates.
2) Power it off, try unplugging the USB and plugging it back in (this will force windows to recognize it a second time), power it back on and verify in the sound that it is not "default" again (it has to be on to verify this though)
3) If it has a software interface with it, pop that open when it is on.
4) Try it in ASIO mode on a stand-alone plug-in if you have one and see if it has the same issue.