Lucky you chose to ask at Sound On Sound forums. In the Production forum there is a sticky
thread for our very own Mike Senior's online library of free to download multitracks.
Here's a handy link
that will take you straight to the thread.
Mike's post will tell you more, but the highlights include:-
more than 50
songs that Mike has mixed for the SOS Mix Rescue column, so you can read
Mike's articles and learn exactly how he handled the mix.
wavs not mp3s
Largely, but not entirely, dysfunctional.