So... eliminate Sonar. Can you listen to youtube through your interface? Volume nob isn't at 0 is it?
Also, lots of people sometimes accidentally mute tracks or clips by accident. Muting the master bus could mess you up. Or sending the master bus to an out that isn't routed to headphones or stereo.
I suppose at a computer level, it is possible to disable your USB bus. That could make it so the interface doesn't work, but then the interface wouldn't show up in Sonar and you say it does.
Hmmm. Those are my ideas for now. Good luck!!