Reaching higher on the shelf, I find the following in the 800-page _American Folklore: An Encyclopedia_ (1996):

"Folksong: Traditional sung verse that exhibits characteristics shared by other kinds of folklore."

But "folklore," since its coinage in 1846, "has acquired several new connotations and faces competition from such terms as 'folklife,' 'expressive culture,' 'traditional culture,' 'verbal arts,' and 'vernacular culture.'"

Seems to me that just "expressive culture," "vernacular culture" and "verbal arts," taken together, should cover everything from Eskimo walrus chants through tall tales to "The Leggo Movie."

Thus a folksong, according to some scholars, is almost any song that shares unspecified characteristics with any of the above; or all of them, if you're clever enough to find a common denominator.

And I did say "scholars," not "downloaders of mp3s."