First, check your routing. Expand the audio tracks and see what the Output (O) is set to. If 'None', is there anything selectable?
If the Output is set to a Bus (Bus, Aux, Master, etc.) check that bus's output. If 'None', is anything selectable?
Click Options at the top, then Audio.
What is selected as the Recording and Playback Timing Masters? That would be your sound card. If nothing is selected, is anything selectable?
Also, SONAR 4 is a very old program and may not work well with Windows 7. Did it come with a hardware purchase? If so, what hardware?
Edit: subtracted a 1.