I know I'm continuing thread drift here, but I would hazard a guess that as long as humans have had opinions about the world around them they have expressed this in song. That's not to say that all folk is political or that politics is or should be the only subject for a folk song, but I reckon it's one of the many strands of human experience represented. To back my thoughts up I offer this possible evidence: English Rebel Songs 1381–1984. Others may come up with something more substantial.