SOLVED: Always happens - you spend ages trying to fix a problem.  Finally send a frustrated forum post and almost immediately find a fix.  lol. "Edit > Preferences > Audio - Driver Settings and make sure the sound card is selected in the Playback Timing Master list."
 
Been running Sonar X3 with no issues for months.  I got back from holiday and the audio driver had dissapeared in X3 (though Windows was fine playing sound through it).  After a few attempts with the Preferences I reinstalled the driver (SPS-25) and Sonar happily picked it up using ASIO.  Away I went, or so I thought.
 
Now whatever file I open I get no sound - or even meter readings from any audio channels.  Midi meters are showing output and the routing is all correct.  Nothing going from the midi to the synth, nothing from the synth to the Master output or any of the Busses. Nothing going from simple bounced audio tracks either.  New files, existing files, everything.
 
Anyone got any ideas as this is driving me nuts?

If it disappeared out of the blue I would recommend running chkdsk /R from an administrative command prompt.

Hello...Not sure if you did this?...Check the output on the master track is set right, to  main outs 1 and 2.
Sonar sometimes looses the ASIO setup...in some cases, and then check all your tracks to make sure they are routed right if the master is ok.
Then press the play ...see what cooks.