Re: Software recommendations for "mashups"?
If you have a Mac, the included Garageband lets you easily slide/line-up vocal tracks against instrumentals.
I've used Acid and ProTools, but I prep and slice vocals in Sound Forge before dropping them in.) Of course, none of those are free or included with the OS.
Audacity is indeed free, and while I don't like it as an audio editor, it's great for decoding/converting certain audio files.
There are lots of great multitracks, RockBand tracks, and 'pellas out there... just look around.