I lowered the volume of a track with the volume control in the track view in Sonar. While actually moving the control, the volume went down, as it should.
BUt--when I let go of it, although the control stayed at the same place, the actual heard volume went back up. Even when (as an experiment) I set the volume to 0 (the control remained at 0), the actual sound was loud.
I sense this is a GPO problem rather than a Sonar problem, because it doesn't happen with other synths.
As an alternative, I tried lowering the volume control for that instrument in GPO itself. It didn't stay where I set it, however, but bounced back to the middle. (In any case, I'd rather control the volume from Sonar.)
Usually if you make the track with the Sonar Synth rack, it creates 2 tracks. A midi track and an audio pass-through track (which you can freeze if you want to). If you're adjusting the volume in the midi track, try doing it in the audio track instead. Another possibility is to insert a mixing bus, send the output of the audio track there, then manipulate the bus volume. This works better if you have a bunch of tracks getting the same treatment.
Do you have CC7\CC10 option enabled in GPO? This needs to be turned on to use CC7 Volume in Sonar. Personally, I never use this and set the general level in the player which Sonar will remember when you save your song. Then I control all dynamics with CC1.