This is so wrong on many levels but I have a quick bodge for squeaky belts on a mk6 with an auto tensioner.
1. pour water just before the tensioner, stops making a noise proceed to step two... still making a noise? outside the remit of this post.
2. check outside of belt to see if its shiny. yes proceed to step three. not shiny then no idea
3. the Bodge... take a sanding block and run it on the back of the belt for a min or so until the noise stops
You could buy a new belt and tensioner like I have and then find out your bolts have all rounded off and cant be arsed to do it in the driving snow and rain... or bodge it until it gets better weather. at your own risk but for the sanity of that bloody squeak...