I had a squeaky serpentine belt a couple of years ago. I used belt dressing stuff and that made it worse, as well as got all over the place. The mechanic used a stethescope type thing to listen and thought the noise was coming from some bearings or whatnot. Eventually, about an hour and a half later, he kinda got this look on his face, walked over to his work bench and grabbed a water bottle. Poured some water on the belt while it was running, and the squeaking stopped for about a minute. Did it again, and it stopped again. So, we ran to NAPA and picked up a new belt and problem solved. He was pissed that he didn't try the water first...would've saved a lot of time.
Apparently, he said that sometimes mechanics will get antifreeze or something on the belt that causes it to squeak like that. The water test works most of the time to ddx the problem....