I think your problem is to do with the A2DP profiles of the BT.
From memory the Ax doesn't have the correct profiles to use a BT headset built in. I don't think Lenny added them either.
I'm very mobile at the moment so searching isn't really possible for me, otherwise I would find the relevant threads for you.
Try searching for A2DP, Widcomm stack, BT headset, BT profiles, etc
The answer is here, I just can't remember what it is.
Hopefully someone with a better memory than me will drop in in the mean time.
I'm not even sure if it's the A2DP or another profile.