Squeeky belts are caused from being loose. The belts slip across the pulleys making that horrible sound. What comes to mind when thinking about why it does this when cold and not hot. I would compare it to race car tires. If the tires are cold they do not stick well and once the tires warm up they gain traction. I hope this was a good analogy!
V type belts are designed to ride on the sides of the pulley providing a lot of contact area. If the sides of the belt are worn, the belt will contact the bottom of the pulley and will squeal because there is not enough contact area to hold to and of course the belt could be loose. On Toyotas, the belts would get noisy and a tire crayon would cure it temporarily but when doing this don't loose your fingers. Hope this helps