When you say "folk music," most people think of singer-songwriter things, which tends to reflect an urban, middle class sensibility and does not sound very real to many of us. On the other hand, "roots music" is substantially broader than "folk music" and blurs over into bluegrass, at least old country, and probably R&B and things line that. Some of which is clearly commercial and not usually thought of as "folk," but which still sounds down-home.

The "O Brother" disk includes "In the jailhouse Now," which I've always thought of as country-western rather than folk music. Good song though. On the other hand, "Didn't I Leave Nobody but the Baby" doesn't sound like roots music to me, it just sounds phoney. Not to say the rest of the songs don't sound great.