Well, as a british colony here in new zealand we drive on the left side of the road with RHD vehicles (fortuantly most of our cars come from japan nowdays so the indication and washer controls are on the correct side unlike the poor british).

Anyway regardless of all other considerations i think it's would be best to have the volume on the left hand side for RHD cars. With it on the left your arm just stretches straight out to adjust the knob, with it on the right hand side you have to have your arm in a half bent-avoiding the steering wheel position (in my car at least).