I have a 2009 A6 and heading home from work this afternoon (with the sound low) my stereo just stopped working. After about 10 minutes the power to the factory system came back up but I have no sound. The system appears as normal, I can select radio stations, change to my CD, aux. input, even connect with my phone, but absolutely no sound. It simply appears that the volume needs to be turned up, but that isn't the problem.
I pulled the all fuses in the trunk but didn't find any out, and even the one on the passengers side that the manual indicated ran the MMI stereo system. Any thoughts before I go hand a paycheck to the dealership to figure it out????

 

Have you had the car serviced recently that required the door panels removed? I had a problem with my car that amplifiers would just straight up die at random. after going through two amps I decided to start poking around and realized that someone had driven one of the door panel screws through a stereo wire. judging by what you have said it sounds like a possible short. and my problem caused the exact same issue. head unit is fine but no sound. Might be worth a try to look up some videos on how to remove the door panels?