This happened to me yesterday. Lost audio in the middle of a trip, gps was on the car mount. Back at home I carefully cleaned the contacts on the bottom of the GPS, and on the mount, with a cotton wool bud and some contact cleaner. Audio returned immediately.
I think I traced my problem to a s/w or setup fault. I can't quite remember what I did (to cause the problem) but it was some variation on pairing a BT phone with the Zumo in the car and the way that I achieved the pairing.
I don't have a Zumo but ISTR reading elsewhere that there's a separate volume control buried in menus for the output via the mount/pins. Worth a check.