The Library of Congress is one of the most splendid resources in the country—which is terrific, if you're in DC. For those who aren't (and even who are!), the Library's putting a massive audio archive online, for free.
The "National Jukebox," available on a streaming-only basis, unfortunately, is a massive trove of audio recordings. Music, speeches, humor readings—spanning decades of American history. The original words of Teddy Roosevelt. "Rhapsody in Blue" with George Gershwin on piano. Serious national gems. And, due to some cuddling with Sony, the label's entire pre-1925 catalog will be accessible, encompassing a significant (and widely forgotten) musical past.
Accompanying the huge sonic repository is a ton of album and label artwork, as well as biographical information on artists (which you'll probably need for artists so dead that Sony gave them away for free). Right now the only thing I can hear is a million history teachers across the country, filled with glee. And an opera from 1919. [Library of Congress]