you can use the javascript console for a precise volume input, try
whereas 2 is equal to the volume slider at max and 0 is equal to the volume slider at min
(see https://docs.google.com/document/d/1PBxf...63BmA/edit for more info)
Code:
audioSystem.setInstrumentVolume([instrument id], [volume])
(see https://docs.google.com/document/d/1PBxf...63BmA/edit for more info)