matthewr
spɹɐʍʞɔɐq spɹoɔǝɹ ɹnoʎ sʎɐld
Been listening to this since the eighties...this is from the ethnomusicoligist Marcel Cellier's " La Mysterie de la Vois Bulgare" another great recording by him is "Les Virtuose Roumain", of an all star concert of instrumental music.
One thing he doesn't dwell on in the video is that they sing with an open throat, and without vibrato.
The second comment on the youtube video from a Romanian was super insightful -- apparently four in Romanian has three syllables and counting 1-2-3-4 gives you a complicated time signature with nested tuplets!