Hi Bob, the reason for the low volume is the fact that Garritan uses the mod wheel for volume and when you do something that stops Sonar like going back to measure 1 or to any other measure Sonar will reset all MIDI controllers to zero. That is unless you go to preferences/project/midi and unclick zero controllers when play stops. When you unclick this your controller settings will remain the same, thus your volume will still be loud.
You other problem is a panning problem so check your pan setting in both Sonar and Garritan.
I hope this helps.