An old catskinners trick was to use a bit of ajax on the cables of the old cable blade controls, makes the cable brake grip better and stops the cable from slipping.Maybe worth a try on the belt.Just a bit on the inside of the belts and see if that stops the noise. Another point to check is spray a little wd 40 into the pulley on the power steering, not to much or it will splatter all over the car, just into the pulley bearing.Have the engine running to do this and watch out for the fan. I have done this before and it has removed a lot of noise which normally sounds like slipping belts.
Don