Fred, I'll chime in and correct you on Moby. Moby, like Sam, issued the samples of Lomax's field recordings initially without permission. Like Sam, he didn't expect much success, and he assumed that because the songs were in the public domain, the performances also were. The Lomax organization rang him up and he immediately agreed to pay all the appropriate royalties to the families of the singers.

Jim, with respect, you're worrying too much about Sam registering copyright on his recordings, whether he did or didn't. If he did, the copyright would still only be valid on the parts of the recordings he created: the accompaniments. The songs are public domain, and the performances, since already published, already had rightsholders. Those rights don't get transferred to Sam. So the singers keep the rights in their performances, the songs remain p.d., and Sam only has rights in the accompaniments. If Sam did want to register copyright in the accompaniments, and they don't exist in any other tangible form (like sheet music), the only way to do this is to register the copyright on the new recording as a derivative work. He still only has rights to his own work. Copyright is a bit of a blunt instrument in that sense. (In the U.S., these rights are actually automatic, whether registered or not, but registration makes it easier to prove your rights if they are challenged. I'm not sure how that bit works in the UK--or France.)

Also, you made an analogy to drawing a mustache on the Mona Lisa. Not to be a nitpicker, but the Mona Lisa is in fact in the public domain. Sam would be legally entitled to make a copy of the Mona Lisa, including a photo, and alter it in any way he wants. But he's still not entitled to do that to your field recordings. I point this out merely to show that the principles involved actually aren't self-evident to many people, and it's quite credible to me that he just didn't know the issues.