Actually, she does sing one song during a montage of sorts and she sounds pretty good. Yeah, I love the Amber/Velma part! I have a weakness for mean characters becoming good.

As for Mama, I'm a Big Girl Now, it really wasn't noticable. I love the song, but I don't think the film really suffered from cutting it out.

Definitely go see it! I think it's one of the better stage to screen transitions in the past years (though it started out as as film and, therefore, lends itself well to film).