Apparently this java app was made in 2008 that can beatmatch and tempo match amazon.com's 20 second preview clips. The search is wonky, not returning results for lots of things and it seems to fail a lot, but still

This is one I made. The first mix is off, but the rest.
http://nic-tvg.thisismyjam.com/?20100417005411

I would pay for something that can do this with my mp3 catalogue and yet seems to have not been updated in two years.