FWIW, I also have this problem. I was able to "fix" it by toggling BT on/off/on whenever I get into my car. As you might expect, this causes a connect/disconnect/connect, and the audio problem is fixed (for me, at least)! jmonahan and I have the same car, so this may just be a fix for us, but you may want to try it!
To simplify this process, I've been working with the developer of Bluetooth Power Profile (in the market). This app's base capability is to turn on bluetooth when you plug in power. He has added a setting for we Hero/2.1 users with issues, that automatically does the toggle when you plug in power.
I am not affiliated with the dev. I am just very impressed with how responsive he was to my request. The app has a 10-day trial, and is $1 after that. Well worth it for me. Your mileage may vary.
Maybe this fix will work for you guys?
Eph