WDI
Sounds like a conflict of sharing drivers between programs. There are several ways to avoid these conflicts. But I've always found the easiest way is to configure windows to use the onboard sound chip of the mother board and configure Sonar to use ASIO with my Edirol or Fireface.
This was Plan B. I was trying to avoid it simply because I'm trying to minimize the number of cables leaving my computer (and going into the mixer).
Another reason to avoid it was that I am often comparing my projects with popular songs, sonically, for mixing purposes. So if I've got, say, Adobe Audition or WMP playing something, I like to be able to switch back and forth between that and Sonar X1b. Just to compare lows, mids, highs, etc. and it would seem to me that if one of those two was using a different "soundcard" I'd be comparing apples and oranges instead of apples and apples (ie, both using same card).
But definitely Plan B if I can't get this resolved. I just don't know why I can't get Sonar X1b and WMP to play at the same time on the same card if "Share with other applications" is checked.