Reason won't play when it has focus.

I set up Reason 5 a long time ago through Sonar X1 and it had been running fine. Last time I went to tweak something in Reason, it wouldn't make any sound. I can trigger it fine when in the context of Sonar, but right when focus goes to Reason I hear nothing. This is problematic, as you may imagine, because I need to audition sounds, and make other mods while I'm playing my midi-keyboard (through Sonar). As it stands, I have to pick a sound, go back into Sonar and play a key to hear it.
 
Anybody got an idea of what could be going on here? Thanks!
-WS