It's quite uncommon for the GPSR itself to have problems creating or reproducing sound.
Here's what I suggest you do to determine if the problem is with the Garmin hardware, or if the problem is being caused by something else further down the line, such as your Autocom. I don't want to say bad things about the Autocom, but I have seen a lot of complaints from Autocom owners who have hooked GPSRs up to the Autocom - and very rarely seen sound related complaints from people who use the GPSR on its own, with just the parts that come in the box from Garmin.
1) If you have a Garmin 12 volt power cable with a speaker in the end of it - such as what you would use in a car - then plug that cable into the back of the Nav III, stick the other end into a car cigarette lighter, and test the unit out in your car. If the sound is OK coming out of the little speaker that is attached to the cigarette lighter thing, then there is nothing wrong with your GPSR.
2) Failing that, stick a monaural headphone jack into the receptacle on the bottom of the 4 button BMW motorcycle control cradle, and listen to determine what the sound quality is like coming out of that jack. Again, if it sounds OK there, then there is nothing wrong with the GPSR.
3) Find someone else (friend, fellow rider, dealer, etc.) who has either a BMW Nav II or a Nav III cradle installed on a motorcycle, and put your GPSR into that cradle and see what the sound is like. If it sounds fine on someone else's motorcycle, than the GPSR is blameless. Note that the cradle for the Nav II, Nav II+, and Nav III is the same, so, you can put any version of the BMW Navigator into the cradle (except, obviously, the first version - the Nav I).
Be aware that there is a difference between the car audio 12 volt plug and speaker gizmo for the 27xx and 28xx series of GPSRs vs. the original 26xx series of GPSRs. The 26xx did not have 'text to speech' capability (meaning, it did not pronounce street names), and it had a slightly smaller speaker built into the cigarette lighter power plug. The 27xx and 28xx GPSRs - including the BMW Nav III, which is basically just a 28xx - have a slightly larger speaker built into the 12 volt cigarette lighter plug. This slightly larger speaker is needed to reproduce the higher quality sound that the text to speech capable GPSRs produce.
If you use a 26xx speaker (power plug and speaker combination) with a 27xx or 28xx GPSR, you will notice a slight degradation of the sound quality, although it will still work OK, and most people who are not aware of the difference between the two speaker sizes will probably not even be aware of the degradation.
Michael
Has anyone ever had sound problems with a Bmw navigator, Recieved my new NAV3 Today and the sound is crackling up to volume setting 60% and then 60% upwards it fails completely. 
