I know many here are aware of this, but just wanted to share how actually disabling the network card in device manager fixed a lot of strange audio glitches and drop outs.
 
A new issue I'm hunting down is that I'm finding the "play" vsti from east/west goes silent sometimes. It will show meters on the track, but no audio comes out. Restarting Sonar fixes it, but it's strange that it will just stop playing audio. Not sure if any other vsti's have this same issue.
 
This is JP update.

Do the instruments you have loaded in Play respond to CC1 or CC11? I always place a CC1 and CC11 at the start of each section of a song to be sure they aren't set to 0 (and therefore silent). Just a thought.

CC1? Isn't that set to "modulation wheel" by default in most synths? CC11 is (usually) "expression", which might be volume but could be pretty much anything.

Volume is usually (but not always) CC7.

 
Garritan software uses CC1 for volume so that is probably where this is coming from.  Also if you are using and/or your VSTi is set up for a breath or wind controller then CC2 will also affect volume.  Thus CC7 and/or CC2 affect volume while CC11 affects it on most soft and hard synths.

A lot of east west libraries have certain instruments that use cc1 to control volume, e.g. Hollywood Orchestra.

My point is to check for any CCs, keyswitches, etc. That could cause the volume to be set to zero.

Could it be that something else has grabbed your ASIO audio drivers away from Sonar?
 
Bob Bone