iOS 10.3 and Audi, Bluetooth audio no longer works. Anyone else?
Well, Aren't I a fool! It seems that the iOS update for some reason decided to reset the system volume to zero (but only when connected to that Bluetooth device). 

The apps that I used to test didn't have the volume slider visible on screen and the car volume controls don't 'pass-through' so it only appeared silent. 

Adjusting the iPhone system volume via the physical buttons on the phone brought the audio back....