A belt squealing is usually caused by the belt slipping even a little bit on the pulleys that the belt wraps around.
It could be that your belt is stretched and needs replaced or you need to tighten your belt by tightening the belt tensioner.
On most newer cars though the belt is tightened automatically using an automatic belt tensioner that constantly puts pressure on the belt and it could be that the tensioner is bad and not keeping the belt tighten enough like it should.
If the belt is in good shape and not stretched then it could be the belt tensioner going bad and not adding enough tension to the belt due to a bad spring in the tensioner as I've had that happen to me on my truck before.