Hi I have the same on a 1998 A3 I found this on a website in relation to the microprocessor number not sure if the number is correct but someone was looking to buy one for the same problem.
"Hi I'm trying to buy a new micro to fix the audi concert volume control problem. It is fitted with a MC68HC05, but I can't find these on sale. What other chip can you use and do you have to use a new one? from website http://forum.eserviceinfo.com/archives/99999947.html
"Hi I'm trying to buy a new micro to fix the audi concert volume control problem. It is fitted with a MC68HC05, but I can't find these on sale. What other chip can you use and do you have to use a new one? from website http://forum.eserviceinfo.com/archives/99999947.html