A quick test just to verify a slipping alternator belt. Take a bar of face soap. Rub it on the v portion of the belt or belts if there are a tandem pair driving the alternator. With the engine off of course.

Squeek stops then they were slipping. The belt or belts v wears narrower with time until the belt bottoms in the groove. This senario can also make it howl.

Or if a good glaze is present it lessens the friction ability. Then there are good belts that have just elongated or stretched a little with time. Adjust them.