The only way to deal with this is to buy a replacement, there is no simple cheap fix. Its a common fault. Check out ebay, there is a healthy trade (and price) for an audi concert. Be careful if you swap to an aftermarket though, there are issues with the rear speakers (which have their own amplifier) and the aerial (which also has an amplifier). Neither of these are a major deal and there are ways around them, but you need to be aware if you do change.
I had a nice Alpine hu with MP3 changer in my wifes 1.8t sport. This had a fosgate punch amp powering the rear Calibre replacement speakers. A good setup which I must get round to putting up on a ebay one of these days!