Hi , did you get this sorted. I had this issue a few of years ago with intermittently losing all sound to eventually no sound except on frosty mornings. Sent my MMI to a third party e-bay repair service and it was repaired and returned back in the car in 72 hrs. Just search Audi MMI no sound repair and you should get a few listings up. I think the cost for me was £175.00 about 3 years ago and had no problems since.