The key is just to ignore the pain because physical comedy only works if you see someone get hurt and they aren't actually hurt. If someone gets hit in the face with a bat falls down and gets back up it's funny. If they stay down and their jaw is wired shut in the next scene it's really tragic and weird. You have to pretend it doesn't hurt.
The thing I thought about doing it was it's Comic Relief and you've got to be funny. So although I did try to sing properly it obviously has hilarious results when you can't sing.