folk song vs ethnic music

folk song and ethnic music both are nouns.

Word	Noun	Adjective	Verb	Adverb
folk song	Yes	No	No	No
ethnic music	Yes	No	No	No

As nouns, ethnic music is a hypernym of folk song; that is, ethnic music is a word with a broader meaning than folk song:

folk song: a song that is traditionally sung by the common people of a region and forms part of their culture
ethnic music: the traditional and typically anonymous music that is an expression of the life of people in a community

Other hypernyms of folk song include song, vocal, folk, folk music.

folk song (noun)	ethnic music (noun)
a song that is traditionally sung by the common people of a region and forms part of their culture	the traditional and typically anonymous music that is an expression of the life of people in a community