Is it mutar to listen to "African drumbing", or "Buddhist chants" when they might have been done l’shem avodah zarah? (We know ’maase rav’; that many rabbanim allowed themselves to non-jewish music ’b’derech akrai’ to be ’maaleh it to kedushah’.)

There is a difference between non-Jewish music and music of Avodah Zarah. Non-Jewish music (i.e. comes form a non-Jewish non-idol-worshiping source) can be “converted” into the faith while Avodah Zara music is out of range, as Avovdah Zarah contains also the prohibition of benefiting from. (Yabia Omer 6 OC 7. see ibid paragraph 3 regarding your "Maase Rav")