I'd probably include Jean Ritchie on your list of examples.

I am no expert or student of the the differences and similarities among regions of the central and southern Appalachians. But it seems to me there are subtle differences in quality of voice, diction, ornamentation, and tonality to the Appalachian area of southern West Virginia, eastern Kentucky, and the extreme southwest of Virginia (and perhaps a tiny northeast corner of Tennessee) from that of the mountain voices of North Carolina, southwest Virginia from, say Galax, east, and the extreme northwest tip of Georgia.

I'm wondering if it has to do, at least in part, with gradations of nasal mountain twang and southern drawl.