I was just doing a session today recording some vocals with a friend of mine.  Had my regular rig and interface just set up like normal.  But I started having a problem that was very weird.   Every time I would undo or save Ctrl - S, I would completely lose the playback sound on my Lead Vocal track.   Boom - nothing there, no sound.   So I'd have to restart Sonar (X2), every time that happened.
 
I've never had this happen before, completely foreign to me.   I've checked all my track and I/O settings, nothing was any different than it normally was.

What happens to playback sound if you bypass all effects?  (Hitting 'E' on your computer keyboard toggles off/on all effects).
 
If it is not from one or more plugins, perhaps your buffers need to be increased.
 
Give the above a try and post back, please.
 
Bob Bone
 

OK, I'll try that.  But I can't guarantee that I can replicate the behavior.  Because I was using all of my same gear in the exact configuration that I usually do.  Only different in this case, was that I was in another building using different power outlets.     Otherwise, same gear and same settings as usual.