I used to be set up to do this kind of thing.
(I did a remix of "Croydon Library" by Fudgedubnofunk, and had a stab at something or other by Curve once as well..)
I'd have loved to have had a go at the Shatterproof remix.
Audacity is good for manipulating multitracks, and it's free.. but you'd need something to create original bits as well. I used to use FL Studio. And some tracker program or other as well. (MPTRACK?))
The "challenge" is synching up the "given" bits with your own bits.. oo-err missus!
![wink wink](/forum/images/graemlins/default/wink.gif)