It goes to the next-of-kin, regardless of whether they know about it or not. Depending on jurisdiction if there's no next of kin ownership may transfer to the government.
Who would own derivative works made while it was in public domain?
I create a derivative work (compilation of my favorite chapters from X PD books, including much new commentary from me). The copyrights to that belong to me (in US at least). Then all of the sudden those X PD books are now copyright by X new/previous holders.
What a cluster fuck. Make bunch of lawyers rich arguing over it though.
That's part of the case, if I understand it: some of the plaintiffs are publishers who published sheet music that was at the time in the public domain, but was then re-copyrighted, forcing them to pull those publications.
What unfortunately doesn't seem to be part of the case, but would make for an even stronger plaintiff (at least rhetorically), would be an orchestra that recorded a public-domain work but then found itself unable to distribute its own recording, once it was re-encumbered.
If the stuff was out of copyright, they would revert to the public domain in the US (in other countries, the copyright status would remain unchanged). In short, no one would end up owning the copyright for works where copyright has expired.
That's kind of the problem here. People made derivative works of out-of-copyright materials that they wholly owned. Then Congress revived some zombie copyrights and threw everyone into limbo.
Given that these are all works in copyright _somewhere_, you presumably just have to look at who the copyright holder is in the jurisdictions where the works have been in continuous copyright.
