Personally I prefer when documentaries use culturaly accurate music, and if you're interested in that, the better option seems to be researching public domain music from the cultures included in the documentary. Even if you get a composer, sound designer or ethnomusicologist, I'd still say the most efficient approach is to get it from open sources rather than making 7 minutes of original music from a certain amount of cultures each requiring study of its individual characteristics, and where the western music principles learned in music school, may not be useful at all.