Here it is: T Bone Burnett's best soundtrack yet! Just kidding... though some youngsters might need convincing that the wealth of early-20th-century Southern spirituals transferred from 78s to six CDs in this thrilling set are all for real, not crafty re-creations. Races and gospel subgenres mix (the Stanley Brothers pray alongside Blind Willie McTell, to cite a couple fleeting ''name'' acts), united by a hardscrabble rural existence, dread of sin, and hope of flight. The package too is a thing of beauty, an oversize cedar box that envelops 135 songs and 25 sermons in healthy annotation and actual cotton bolls. If ''O Brother'' was the ''coffee-table album'' of its year, ''Babylon'' feels like the coffee table itself.