It still is reduce-able to the more general blind source problem, right? We can conveniently "forget" that we know what the sources are so now we are blindfolded and can still use the same techniques to solve it.
Sorry I didn't see your responses until now. Indeed, there are many ways to slice the specific problem. I was specifically responding to the parent's statement:
> It would be a game changer if someone were to come up with a novel method of decomposing audio into discrete components
It's something that has been generally addressed and ~works. It will obviously depend on the specifics of the application, and yes if you can constrain the problem space further you ought to do better!