export to mp3

Check the mains level.
Are wave exports OK?
Are the mp3s being tested by re-importing into SONAR?
If all else fails there is http://mp3gain.sourceforge.net/

Thanks for the feedback.  The main out is up to max volume as well.  Not ready for mastering.  Working with collaborators to improve lyrics and melody and easiest way is to send mp3 via email. In the past when I raised the master level I achieved a satisfactory level of the mp3.  Will export to .wav if I have too.

I would also check the mixer in Windows to be sure that nothing has been turned down there.

Hmm, the windows mixer should not be involved in ASIO devices,,,Your Sonar faders are in control of all audio. 
There should also be the interface bus in console view.  Double clicking any Sonar faders re sets them to unity and that's where a master buss belongs. 
If your meters are still showing low then you need to turn all your tracks up to get the master buss meters closer to "0" . Your meter is your guide. 
If the master buss meter is showing close to unity then your export will be at that level, and this should be regardless of format. But this can often result in a song that has a Average RMS level of -22Db. Modern songs are now around -10 Db. I go for -12Db. 
Peak level and average RMS level are 2 different things. 
If this is the case your next move could be to put the LP64 multi compressor or Boost 11 in the master effects bin and push it up a little.  This would be considered mixing and Mastering in one pass but for your needs is fine. I was originally concerned that you where sending out a finished product, not a demo. 
 
Another check is to mute your master buss and make sure the project goes silent. Often track outputs can get directed to your interface and are bypassing your master.