Answer: Recordings of both have been selected for inclusion in the Library of Congress's National Recording Registry. According to the New York Times, Tupak's recording "Dear Mama" was picked as well as a recording of a debate between Niebuhr and John Flynn, chair of the America First Committee, on U.S. aid to Britain during World War II.
"The National Recording Preservation Act of 2000 directed the Librarian of Congress to select 25 recordings annually that are “culturally, historically or aesthetically significant” and at least 10 years old. Nominations are made online by the public and by a group of professionals in the fields of music, recorded sound and preservation. The recordings are preserved at a state-of-the-art facility in Virginia."
Other selections for 2009 include Loretta Lynn's "Coalminer's Daughter," Bill Cosby's "I Started Out as a Child," and the self-titled debut album by The Band. You can see the full list here.
This is probably a good opportunity to mention that Union Theological Seminary's William Smith Morton Library has an extensive collection of audio recordings by Reinhold Niebuhr available for order. You can find a list of the recordings and ordering information here.