This isn't the bluetooth - but a bug on the 'phone. It's telling the car that it needs the bluetooth channel to send audio, so the car obliges by shutting off the radio, activating bluetooth.

Your own number shows as the "handsfree profile" the car's system uses requires a number to be transmitted, so that's obviously the one the phone's developers used.

You'll likely find that if you quit the Google app (by double tapping home, pressing and holding google, then pressing the "minus" button in the task bar) sound will be restored as the phone will tell your car it no longer needs the audio lock.