About the "Voices in Harmony" album, WAV: there are a number of groups (mostly twos and threes) singing in harmony. I'm not sure what you mean when you ask whether they're "all singing just the tune in close harmony" - if they're all singing just the tune, that's not harmony, it's unison. If they're all singing the tune but in different keys (say a fourth apart) that's an old-style harmony called " organum" or "parallel fourths" (or fifths) - it sounds a bit spare and edgy, and it's not usual.

What they are normally singing are conventional harmonies based around the chord that related to each note or each bar. They're not "close" in the sense that barbershop harmonies are "close" - that's something different again. They are similar to the harmonies that would be used if the tune was arranged for, say, piano, or string quartet.

They are very far from being "art-songs", most having quite a rough, earthy sound, and some of the arrangements go back centuries.

What I find fascinating is how very different each duo or trio or quartet sounds, in contrast with classical singing, in which a given setting of a song sounds pretty similar even when sung by diffferent voices.