There is a spray you can buy and it does quieten the belt(s) down. Just go to supercheap or any other parts shop and ask for it (sorry, i forget the name). If the belts look cracked, just replace them, you may as well. Check how much the belt deflects, there are specs to follow, use comon sense if you cant find them, and dont do it up too tightly.

Just so you know when you go to the auto parts shop, and I'll whisper this... (we dont have fan belts, hehe. We have electronic fans. Belts for power steering, alternator, and air con though (if you have those options).)