If the radio is still working then i means cd changer is connected . A6 c6 audio works on fiber optics connection called MOST , it could be no power to the cd changer or cd changer is dead .
There is no dedictated fuse for cd changer only so if everything else is working then its safe to assume the fuse is fine.
Best is to take it out and check with mulitmeter if there is power in the plug
Also if someone messed up with the cd changer it could be componet protection triggered . Do a vcds scan to check . CP removal requirers special tools that audi only have ( i think its £70 to remove )
Hope this makes sense :)