What if....

[ApocaRUFF]

wouldn't it just be a giant mashup.

some things weren't meant to be put together, like pineapple and pizza, and Death In June and Iron Maiden

The algorithm behind it would put together the pieces you find most enjoyable in a way that is pleasing to the human ear, so it's not just a giant mash-up. It would essentially be trying to create something coherent out of a lot of smaller pieces. A further example, through the voting process the website/program/whatever may discover that you like a certain type of melody in the middle of your track, but strongly dislike it at the beginning.

[LastDemon]

Yeah the issue is people have no idea what they want. There would have to be some sort of top 10 list of sound clips to base it off. People would see top 10 and go "wow that does sound good" and go from there.