You're using the wrong terms here. There are two main copyrightable entities in music. First there's the publishing side, which covers the song's actual compos

You're using the wrong terms here. There are two main copyrightable entities in music. First there's the publishing side, which covers the song's actual composition. Second, there's the master side, which, once the song is recorded by an artist, covers the recording. In statue, you can freely cover any song, and there are royalties baked in. This is covered on the publishing side. There's the mechanical, which covers each duplication of the new recording. There's also the performance royalty which is paid every time there's a public performance (on the radio, in a bar, at a venue), and are administered in the US by the Performance Rights Organizations (ASCAP, BMI, SESAC). There are disputes about these payouts where digital and terrestrial radio are concerned, but that's a whole other thing. Checkout some of the recent congressional panels for a crash course in consent decrees and the other wonderful things each side is fighting about (Pandora vs. ASCAP + BMI, mostly). Concerning remixes, this gets into the master side. There are no rights written into statute that concern the master side. This is likely due to the major label's continuing influence over the process. As soon as you take a master recording (or the stems therein) and alter it in a DAW or elsewhere it's transformed into a derivative work. In order for it to be legal to sell this work you need consent from all of the stake holders, that is, you need permission from the master rights holder(s) and the owners of the publishing (original songwriters). This is, right now, an ad hoc process. There is no industry agreed upon standard for payment, mostly it's people making it up as they go. I have a general rule of thumb: if there are more than 3 writers on the publishing side, it's not getting cleared. This generally holds in my experience. Any platform that hopes to solve these problems needs to have end to end consent from all of the rights holders before offering a path to monetization of the derivative, or else you're opening yourself up to litigation. The music industry, as we've seen, is endlessly litigious.