There is something on some US site about fitting new cables, so it can be done. Can't remember which site though. Broken Camshaft maybe?
www.endosquid.com/audi/
Excessive friction (ooh err) is the biggest cause of electric window problems, causing motors to burn out and cables to snap, keep cables and guides well lubed with lithium grease, and I spray silicone in the felt guides too. It is quite noticeable how much more easily the window works after doing this.