Hi Steve,
Great news that your dealer has now fixed the issue.
Unfortunately I haven't had that same customer service from Audi in Brighton, and they have now quoted me 2,995 to replace the MMI Unit after a full inspection. Even though they couldn't actually confirm that was the issue.
The car is 4 years old and they said it's out of warranty and there in nothing they can do. I argued the case that this is a major part of the car which should not be going wrong after 4 years, regardless of whether it's in warranty or not.
The strange thing is, I recently unplugged the USB charger that was in the centre console and it has worked perfectly for the last few days. No sound issues at all across DAB, Media or Bluetooth. Very strange to think unplugging something would fix the issue when Audi recommend replacing the whole MMI System.
Anyway, hopefully it continues to work okay as I can't justify spending 3 grand on a fault that is intermittent.
Thanks for the update though!
Tom