How can I fix my item, Audi A3 1.8T, MK1 1998?

Hi, My volume control has gone on my Audi system, just stays one level, or when you try to increase volume it is far too loud. I understand this is a common fault...How much would it cost to fix? Also is there any upgrade CD system either Audi or Market cd stereo I could get? I have a cd changer and BOSE system so I need it to work with it.