Just a thought, but if you put just a dab (a very small drop) of petroleum jelly on the pulley itself (the shiny part that the belt comes in contact with) that it will raise the coefficient of friction enough that the squeal might stop.
It sounds like it's a dry squeal. The pulley actually spinning faster than the belt is moving and the contact point making a squealing sound. A small (very tiny, like 1/2 a tic tac sized dollop) on the shiny pulley surface smoothed out to cover as much as possible should cause the belt to grab against the pulley and stop the squeal (for the short term) Not so much that you just greased it and it stops the belt altogether.
