The Performance or musical style is never in Copyright.

What is protected for 70 years are the rights of the writers on the words (lyrics) and the music (the melody, not the style!)

This is the legal situation in old Europe where we live and sing your beautiful american songs...

To create new rights and make money even the Carters a wrote a few lines more into traditional songs - the same did Leadbelly and the writers of Johnny Cash, when they invented some of "their" songs from the basis of old material long time ago sung by negros...

Dr. Raphael Baer

(see: facebook)