"'folk voice'"
I agree with that Brian, but it's not that simple
Your voice has a repertoire of sounds which are evoked to respond to certain situaltions
You don't use the same tones to a fractious child as you do when you are persuading a woman to....
The same with singing (if you work at it)
In speech your tones are evoked by your feeling' what you are saying (you don't even have to think about it if you 'feel' the song you sing - it comes naturally - certain emotions produce certain tones - hard, soft, narrow, broad.... I believe that is the case with singing

If you regard you voice as a box of tools that first need mastering until you can do whatever you want with them, then keep then in good condition, you can re-create any type of song you want - the whole repertoire becomes accessible to you (if you wish)

Then comes interpretation, which is far more individual - in our workshops we used to ask our singers to sum up in as few words as possible (one is perfect and sometimes sufficient) what the main emotional objective of the song was - why was it first created?
THere are excercises for both of these aspects

Sure - Traditional singers didn't do this but they didn't need to - the songs were part of their culture - often real 'mothers milk' stuff

The one sing a 'performer' has to do is to choose their songs carefully so they don't all sound the same
If they don't do that, the audience's ears 'go to sleep' and you lose their attention (Charles Parker once researched this using schoolkids)
One of the litmus tests of this is to listen to a solo singer on an album and see how long your attention span lasts - more problematical with longer one-sided CDs compared to the old vinyl LPs
Jim