I was not aware of a "muffled bluetooth" issue. I've used all the betas of 4.1 and have never had an issue with bluetooth.
So annoyed that A2DP isn't fully supported still.
I can change tracks with my bluetooth headphones on my 3g...
no on 4.1... none for that.
There are many mysteries with Apple but this isn't one of them. Apple dev support confirmed this was fixed as of 4.1 beta 3 and dozens of devs and non-devs alike have confirmed with various headsets.
Now BT issues like connection lost and OEM car sync may or may not be fixed but at leaset we know the garbled audio is a thing of the past.
It would be interesting to know if this is model specific problem relating to the muffled sound.
I opened and helped resolve this bug with Apple. It would have been an issue with any BT headset that has active echo cancellation.
I noticed the very same problem with Bluetooth and Navigon. Glad I'm not alone!
I don't think the BT/Navigon bug isn't a universal one. I'm still on 4.0 on an iPhone 4, using a Motorola T505, and while I've had both the muffling and an echo issue, I've never had it cut off any of Navigon's instructions. That makes me think it may be partly a compatibility issue with your BT device. Be sure to let the Navigon folks know what hardware you're using if you report it.