Hey Mtn. Dog-
Sounds like your automatic volume control is going wacky. Try shutting it off. Go to the main menu on the Nav panel, and under audio options hit AVC...take the level down to nothing. I hated the automatic volume control, it would lower my music level to nothing when I got off the freeway, so I got rid of it. If that doesn't work, I'm stumped.
Best,
Joan