If you have p.s. , make sure there isn't any fluid leaking from it. Something has to be getting on the belts/pulleys to persist the squeaking. If they aren't loose.
if your new belts are squeaking there to loose or to tight, you'll know if it's too loose by your lights etc. if it's too tight your gonna burn the belt and possibly burn out your alt, i'd say it just needs a little adjustment if it's a new belt.