I do think the musical tone production has a lot to do with differences in how spoken language or accent uses the voice.

I have a lot of odd theories- they may not be all that useful other than to actors perhaps- about the differences in vocal tone production and the way voice is used in speaking in different cultures.

I'm Slavic (Russian/Ukrainian) and we just sing folk music that way that you're describing . I personally also think some of it comes from how we make our vowels during normal speech in Slavic languages, and what our accents predispose us to do with our mouth regardless of what language we're singing in, along with what is considered culturally acceptable for women to do with their voices.

Slavic women tend to use a much broader range of their vocal range in talking, I think, than American white women do (I'm not sure the same cultural restraints exist for African-American women in the US) -I think that in the US white women are pushed to restrain their speaking voice a lot more, and sound like little girls, than we do in Slavic cultures (or at least the one I"m from), and here in the US we're encouraged to use more of a breathy, higher tone than might be natural. As a contrast, think about black women- you hear a lot more black women in America with low voices or brassy tone to their speaking voice, I think, than in white people, and I'm sure it's just cultural crap that keeps the white women trying to sound like little girls. I hear the same sort of broader range of tones and less 'breathiness' and restraint in Slavic speakers.

I'd ALWAYS noticed a difference between how I sang, from having learned Ukrainian folk songs from my grandmother, and the much more restrained tone production that's common in Western pop for instance.

I've got into singing Southern Appalachian traditional music (ballads and some traditional religious singing) before I learned the Balkan stuff, and I noticed similarity between what I sang growing up, in terms of tone production, and how traditional appalachian singers produced sound.(think the stuff in the movie 'Songcatcher'- they did a good job on the actors doing the singing, who were coached by an actual singer from that tradition)

It always stuck with me that one of the early 1900's collectors who described the Appalachian singing said 'they sang as easily as they talked'- which seems to be the key in many folk traditions- using your voice in a similar way as free, relaxed talking.