That site is wrong: tha’s definately Harry with a wand under that cloak, not a torch. But this was pretty funny:
Am I the only one that loved the chorus? 
After all, it’s a boarding school. They have to have a choir!
I just realised they are singing along to the theme music.
Gah! I meant Joel Grey, not Joley!
All this talk of the chorus just made me remember that part in the first book, the one and only school song that Hogwarts had. Maybe the chorus is supposed to be the movie version of a school song or something.
I just started re-reading Azkaban (it’s my least favourite, so I tend not to remember it very well) and the song that the chorus are singing would be part of the Halloween celebrations, a particularly significant part of the book (many plot points take place on that day).