Call it the Coachella of the folk posse. Early last year, the indie Arhoolie label, longtime champions of every genre of American roots music, threw itself a fiftieth anniversary bash, captured on these four animated discs. The guest rock marquee names include Ry Cooder, who stomps through electrified Woody Guthrie and “Wooly Bully,” and Toni Brown and Terry Garthwaite, who revive the beautifully dusky harmonies of their Seventies feminist band Joy of Cooking. But some of the liveliest and most life-affirming moments come from the purists: accordion-pumped Cajun from the Savoy Family Band and ferocious sacred-steel shredding courtesy of the Campbell Brothers. It’s not EDM, but it is proof that roots music still has a vigorous pulse all its own.