This audio clip is from the Library of Congress event the "Festival of Music commemorating the seventy-fifth anniversary of the proclamation of the thirteenth amendment to the Constitution of the United States," 1940. Below is an excerpt from Faith and Philosophy pages 174-175 about this event:

The National Stage: An instance of Locke’s predilection for a national stage is the Library of Congress concert. On 20 December 1940, the Music Division of the Library of Congress hosted a concert of traditional Negro folk music, performed by the Golden Gate Quartet, accompanied by Joshua White on guitar and vocals. Alain Locke gave the opening commentary on “The Negro Spiritual” and served as the event’s “time-keeper”—probably a euphemism for “master of ceremonies.” Blues and ballads were introduced by poet Sterling Brown, with Alan Lomax as commentator on the “reels” and work songs that the quartet performed. The official program notes cite the occasion: “The Librarian of Congress and the Gertrude Clarke Whittall Foundation present a Festival of Music commemorating the seventy-fifth anniversary of the proclamation of the thirteenth amendment to the Constitution of the United States.” Sound recordings of the concert were made in the Library of Congress’s Coolidge Auditorium in Washington, D.C. and produced by the Music Division and the Recording Laboratory of the Library of Congress.

Jeffrey Stewart has published a transcript of Locke’s talk. Apart from the intrinsic value of his commentary, Locke made a trenchant statement on democracy:

"Now, of course, the slave didn’t get his democracy from the Bill of Rights. He got it from his reading of the moral justice of the Hebrew prophets and his concept of the wrath of God. And, particularly, his mind seized on the experience of the Jews in Egypt and of the figure of Moses, the savior of the people, leading them out of bondage, and, therefore, there is not only no more musically beautiful spiritual, but no more symbolic spiritual than “Go Down Moses."

The CD can be obtained from amazon.com. Here are full archival/bibliographic details:

Josh White, Sterling Brown, Alan Lomax, Alain Locke, the Golden Gate Quartet, Freedom: A Concert in Celebration of the 75th Anniversary of the Thirteenth Amendment to the Constitution of the United States (1940). Compact‑disc. New York: Bridge, 2002. This CD digitizes two monaural (not stereo) sound tape reels: analog, 7 1/2 ips, 2 track, mono.; 10 in. + 1 program ( 12 p.). Catalogued as recording AFS 6092-6095, Archive of Folk Song at the Library of Congress.

On the compact disc recording issued by Bridge Records, Locke’s lecture, "Spirituals," is eliminated entirely, except for his last sentence, “The quartet will close with ‘Travelin’ Shoes” (Track 7). But Locke’s introduction, “The Negro Spiritual,” is featured on Track Two (1:14).