Mumford & Sons have no ancestral connection to the travails they sing about on "Dust Bowl Dance," but Brits have a long tradition of expropriating American roots music. Americana is the London quartet's dominant influence, from banjo-driven bluegrass to Dobro-dusted folk. And there's an echo of such U.K. acts as the Waterboys, the Pogues and Fairport Convention in Mumford & Sons' folk. Their debut LP, Sigh No More, proved an overnight sensation in the U.K. and Australia, where it knocked Susan Boyle from her perch atop the charts. As important as their roots-music allegiances are, though, Mumford & Sons' organ and keyboards separate them from their string-band brethren, recalling the fervor and warmth of the Avett Brothers.