W B Yeats wrote the following about Folk Art, which of course includes Folk Music.

"Folk-art is indeed the oldest of the aristocracies of thought, and because it refuses what is passing and trivial, the merely clever and pretty, as certainly as the vulgar and insincere, and because it has gathered into itself the simplest and most unforgettable thoughts of the generations, it is the soil where all art is rooted"

Now as a starting point that's good enough for me. Of course you can manage to fit much into that broad definition, but it certainly gives pointers as to what to leave out.