I have sound (music) from the Nav IV Media Player if I plug the headphones into the Audio Out socket on the side. And I can connect by Bluetooth between my Nav IV and Sony MW headset and receive navigation directions etc on the headset. But when I am connected by BT and try to use the Nav IV's Media Player, I still get the message that I have to "Connect headphones or an A2DP Bluetooth headset".
So the navigation part of the Nav IV is happy with the headset, but the Media Player part is not!!!!
I have looked at the manual for the Sony MW600 Bluetooth headset, and cannot find any reference to "A2DP", but the Nav IV is clearly happily connected on the navigation instructions side, so I would have thought it would work!!