“Maria durch ein Dornwald ging” is a German Advent hymn from the 19th century. The melody possibly dates from the 16th century (wikipedia). It was first printed in 1850 in “Geistliche Volkslieder mit ihren ursprünglichen Weisen – gesammelt aus mündlicher Tradition und seltenen alten Gesangbüchern”, [August Haxthausen]. Paderborn 1850. https://sammlungen.ulb.uni-muenster.de/hd/content/pageview/1931586
For overtone singing, the challenge is the minor scale melody. Because the singable harmonic series is tuned in major, you have to change the fundamental several times to be able to sing the melody. Michael Reimann improvises his piano part according to the demands of overtone singing. He can do that because he himself sings outstanding overtones. We are so well attuned to each other that we were able to improvise this version freely and record it in one take.
If you want to sing it, you can find the free sheet music in my sheet music collection.