Interested in folk, blues? This is a treasure.

The Alan Lomax Sound Archive Now Online: Features 17,000 Blues & Folk Recordings

among the 17,000 recordings:

Mississippi Prison Recordings 1947 and 1948

"The Lomaxes and other collectors of their time (and also decades later) found some of the most powerful vernacular music of the American South in the region’s oppressive and violent prison system."

http://www.openculture.com/2012/03/the_alan_lomax_music_archive_now_online.html