when should the timing belt be changed
I didn't look up Audi specific information, but on any car with a belt, not a chain, should be replaced at no more than 80,000 miles. I personally would do it sooner that that, say about 60,000 tops. If it breaks, and you have an interference engine that one belt breaking will destroy your entire top end of engine
OK I found a pdf download for your Audi Owners manual and it says "inspect at 65,000km. that's about 40,000 miles and replace at 120,000km. 80,000 miles, which seems a long time to me
http://forums.audiworld.com/showthread.php?t=1961739 --- Check out this thread about changing the belt. I found dozens of belt related threads on that site using the search feature. It sounds as if the belt can be inspected and should be replaced soon if any looseness is detected.