To the original poster:
You posted earlier the following: "Mix knob at 1/2".
The way this works for your interface is that you are with that setting only getting half of the available signal to your output jacks. This is due to the 'mix' level being set at half. I would recommend you consider setting the Mix knob all the way to the right. That position should have a label of PB (according to the doc), and this is what they recommend for Play Back (PB). Making that change should significantly boost your output signal, and will contain whatever Windows or Sonar is sending to the interface (includes mics and instruments plugged in to your interface too, as long as you have them routed through Sonar or Windows).
If you try to do direct monitoring in addition to monitoring the sound output from Sonar, you will hear an annoying delay. By switching the Mix to 100% Sonar/Windows, you will eliminate that delay.
In any case, best of all to your efforts - I hope you are making progress with it all. Once you get it all working, it should remain hopefully stable for you, giving you opportunities to find other things to break. :)
Bob Bone