According to a conversation I had with those good people at Chainspeed (who have a wealth of knowledge and expertise about intercoms, particularly those from Interphone) the problem is that BMW are using a very old A2DP protocol in the Nav V so that it remains compatible with the BMW Communicator Bluetooth intercom. However the MC series of Interphone cannot communicate with this for the music streaming. And until they -BMW- decide to update this A2DP protocol the problem will remain. I note that Interphone were very quick to release updates on the MC series when it initially came out but have not done so since. This suggests to me they are quick off the mark to address issues over which they have some control but are not able to address this problem as it awaits a BMW fix.
Peter