Alternator belt not spinning sounds to me like either the alternator itself it bust or it's adjoining parts i.e. pulley/bearings/belt
If the alternator is bust and it tries to turn, it can give off an awful noise that would scare all the cats in the neighbourhood away.
It could have been sitting up for an awful long time and then it just seized but if it's running a few hours and the belt is moving just fine then I would keep record of it with the dealer and ask for a full guaranteed refund if it comes back!