The John and Ruby Lomax 1939 Southern States Recording Trip, discs 2600, 2601, 2602 & 2603. 1939. (on mp3, from the Library of Congress website)
2600-2602 are songs from a Mexican Christian drama, “Morir en la cruz con Cristo, o Dimas, el buen ladrón.” It’s all pretty boring and irritating, probably because the singers are terrible.
2603 has a couple folk songs by the above singers, and also a few prison recordings made a month later: some field hollers and a barbershop-ish gospel quartet which is quite good, “Ride On, King Jesus.”