The movie's soundtrack is provided by Neil Young. Pretty wonderful stuff.
Today, I came to learn that the song (Prïtourïtze Planinata—Song From the Thracian Plain) is traditional in Bulgary. When Bulgarian music and singing was showcased on an album released in the U.S. back in the late 80s, it made quite a splash, owing to charming peculiarities of Bulgarian vocals.
Evidently, the album to get is this one: Le Mystere Des Voix Bulgares. The video above features that album's recording of "Song From the Thracian Plain."