I had the same issue. My C340 fell of the dash and the volume control (potentiometer) broke off and was rattling inside the case. I tried to find a replacement (103B stamped on the pot) but have not been able to find one. I tried a mini 10K ohm pot from Radio Shack, but the posts were not properly positioned for the circuit board inside the Garmin.

I took the easy way (for now) and simply strapped the two control points together with a piece of wire giving me maximum volume. Orient the circuit board with the missing volume control to at the bottom right. The control mounts with 5 points, two on the right edge of the board are simply physical mounts. There are three other connection points just to the left in a slight arc. The top and bottom point have holes to place the legs of the pot. The broken off legs may still be in these holes. Solder a piece of wire between the top and bottom points. Be careful since the solder pads (to the left of the holes are very delicate.

Not an elegant filx, but you get the voice back.

Jim H