There was a post a few days ago about this song and someone thought that the cast did all sing their own little parts in the song. If that is the case then I thought that Sid had a good voice too.
Have a look through the other post about it and you can follow some instructions toget a MP3 copy of the whole song.
My friend is related to the chap who plays Sid and he said that He and the rest of the cast had singing lessons beforehand so it was definately the cast themselves. - Make your own mind up if the lessons helped.