There are too many to attempt any kind of list. I'll pick out a couple of unusual ones. Mick Jagger sang "The Wild Colonial Boy" in the Ned Kelly film. Marlon Brando sang a snatch of "Long gone Lost John" in a film I've been trying to identify for years, and nobody remembers this except me, so if you could add the name of this film here I'd be eternally grateful.
    An interesting list would be the number of films that have got a fiddler playing Soldiers Joy in. Hundreds, I would think!