This is by no means meant to be an exhaustive list of audio resources, but should provide a good sampling of useful starting points.
The Audio resources linked below offer collections of historic sound recordings made throughout the twentieth century. Most of these recordings are offered as free downloads that may be used for personal and academic purposes.
Topics included within these repositories cover popular entertainment such as classic radio programs to pivotal points in American history such as presidential speeches and proceedings of the Supreme Court.
Be sure to review the copyright information on each of these websites before utilizing any of the content found there!