Yesterday I replaced 5 of the 6 CDs in the changer (though this probably has nothing to do with the problem....).
About an hour later, the sound went out on me when I adjusted the volume with the small control **** of the MMI. I had to fiddle with it and it "crackled" on a couple of times then went out "permanently".
The Mute is "set" according to the MMI display, though I've tried to press the small control **** many times to shut it off....Also, I've tried turning the MMI on and off, the car on and off; and changed to different modes on the MMI, and nothing seems to work to remove the Muting (!?). Also, the switching between modes is a little sluggish (especially to and from CD).
It's essentially a "cold boot" of the MMI system, whereas the volume **** is a "warm boot."
The only way to hard reset the system, AFAIK, is to navigate the menus to "restore factory settings." But I don't know if even that wipes out your phone book, nav locations, radio memory, voice tags, BT pairings, etc.
This sounds like a MMI control unit issue or radio amplifier issue. The MUTE icon will permanently come up if there is a problem with the sound system. In rare cases, we have seen issues with an MMI button stuck down causing "slow" reaction of MMI menu function or permanent muting of the audio system.
First check the operation of all the MMI buttons to see if they move freely and make an audible sound when pushed. If they seem to be working correctly, try pushing down on the volume control **** while turning it. On rare occasions, we have replaced MMI control/button assemblies due to failed rotary switches, but that was mostly the main MMI control ****.
If you can email me your VIN, I'll check the vehicle for TSB's, but as far as I know, Audi has only released four on MMI/Radio issue's and none of them apply in this case.
Hope that helps! If you plan to take it to your dealer, please make them aware that this happened after loading new CD's into the changer and you have manually reset the MMI with no success.