I have purchased a brandnew Audi A8 in November 2005 (the face lifted model - "single frame") here is the real story: No MP3 whatsoever possible !
I decided to do my own thing again and port my old BMW CarX to Audi - here is the result:
check out http://www.carx24.de/carxa.htm
It is the latest Audi A8 layout. Supports MP3, Video and Speed trap warning - absulte same look and feel as the Audi original. PC video and audio plug into the TV-module, and a steering wheel knob integration already works as a prototype in my car. All details explained on the website
Here is how I cabled things (actually had them cabled by Audi...)