I sometimes have trouble with a particular plugin that I mainly use on the master bus. It will not let sound through it sometimes. If I disable it, sound is instantly audible. Have you tried disabling all the plugins just to make sure one of them isn't your culprit?

You could try starting the project in "Safe Mode" and just see if that solves the problem.

Try bypassing your FX - press E on computer keyboard.

Don't know if it's been suggested or not, but have you checked your main outs in Sonar? Select your master bus and look in the inspector.

Have you got any hidden tracks which might be solo'ed?

Bristol_Jonesey
Have you got any hidden tracks which might be solo'ed?

+1 ... that's a good one to check
 
are no meters showing anything or is it just a silent master bus (it could be as simple as having a fader down all the way which can happen e.g. with multiple CS - had that problem once that the A-PRO got stuck and kept sending MIDI messages which moved the faders that were off screen)
 
try another Sonar project and see if it happens there as well (if it does, it's set-up related - if not, it is project related)
 
could be as serious as a corrupted project (I got one at the moment that doesn't play but all meters are jumping) ...
 
 

Is another audio program active (iTunes or whatever)?  Sometimes those programs will "capture" the audio drivers and not share them with Sonar (and vice versa).
 
I always run Sonar without any other audio programs active.

I would create a new audio track and try to record some music on it. If you can do that and play it back (make sure you Solo it) then it would seem to be at the track level. If that's the case you would need to go track by track to find the offender. If not then I would try auditioning other projects. If they play back then you know it is only one project. If not you have some issue with your soundcard and Sonar.
 
Regards,

Hey I just had this same problem.  It was pro channel and the concrete limiter on a master bus.
 
I turned the Pro Channel off and audio would go thru.  Deleted the Concrete limiter and everything was totally fine.
 

Great responses guys!  In response:
-multiple projects (5) tried in X2 AND X3 same result
-no meters moving
-no hidden tracks
-bypassed all plugins
-no other audio programs running
-reinstalled audio device
-audio engine on
 
I have another audio device that I have used sparingly (a Zoom R24) but it did work well with X2.  I'm going to hook it up and see if I have the same problem.

One more question that might be important.  When I check the ASIO panel in audio preferences, under the usb tab there is no device listed.