I have the same problem. I've found a temporary solution in that I always listen to my music at the same volume when riding (2 clicks on the main slider which equates 30% and then 70% in audio manager). So all you have to do is match up your volume preference in windows mobile with the option inside Run.GPS (if you go to the voice prefercene screen there's an option there to set the volume, by default its 10% or something really quiet!)