Every document and voice recording on this site was found through the Library of Congress, whose general collections can all be accessed through https://www.loc.gov. That being said, the site itself has many partner sites and collections that might be helpful in searching for specific content material. Below I’ve listed several links that I found to be particularly well organized and helpful.
World Digital Library:
Library of Congress American Folklife Page:
The Walt Whitman Archive:
Civil War Glass Negatives:
Library of Congress, Books that Shaped America:
Library of Congress, The Language of the Land: