Sure you can edit a finished, mixed song, but it takes a lot of skill, and realistically no one's going to do it without stems. If a song is an executable, it can expose user-friendly adjustable parameters. So one song could have infinite variations. A pirate could record one of those variations and share it, but that's much less valuable than the executable / stems.
What kind of adjustable parameters? I'll admit I'm intrigued by the prospect of songs that vary slightly on each play-through (e.g. slightly different drum fills, different solos, etc), but I suspect this is distinct from what you're suggesting.
I'm also of the belief that piracy of digital arts is largely a cultural thing & that attempts to prevent it by force will never be more than marginally successful at best. That said, I have zero evidence to back it up. It's something I'd like to investigate, but I don't know how.
Around 10 years ago, Digimpro came out with something similar; they introduced a custom audio format and a standalone player that let you change playback parameters and switch between alternative tracks: http://web.archive.org/web/20051215034341/http://www.digimpr...
Don't know what happened to their technology later on.
