usuall squeal will happen beause it is misaligned or loose/tightbelt..
but seems like you said you tried both tightening and loosening, next thing i would look at is if the alternator is even aligned.. try look top down to see if the pulley is straight in line with the other belts and pulleys..
happened out of no where?
might even just be a cheap enough fix to replace the belt.. but if its hot then my guess is its slipping real bad, or misaligned..
also far fetched, but last thing would be that the alt is frozen, but you would know because the car wouldnt be charging the battery at all and you can probably just check that through a multimeter..
but try taking off the belt and spining the pulley with you hand, there should be only a little bit of resistence.