This is a growing list of legal sources of music and audio that can be used in podcast production. If you have a link to contribute, just post it on the network, and the editors will add it to the list.
Archive.org Audio - User-uploaded recordings of everything from alternative news to field recordings of everyday life in Indonesia, and everything in between. Free to download and reuse at the user's own risk.
British Library Sounds - Huge collection of digitized sound recordings, including music, drama and literature, oral history, wildlife, and environmental sounds, some of which are available for download and reuse.
European Space Agency, Sounds from Space - Small collection of space sounds.
Magnatude - This is a record label that works directly with artists. The collection is not free, but non-commercial podcasters can become "download members" (a one-time $299 fee), which allows them to download and use any music on the site in their non-commercial podcast without purchasing any additional licenses. The label takes a 50/50 split of revenues with the artists.
NASA Audio Collection on archive.org - The historic audio record of the history of human spaceflight at NASA, digitized from original reel-to-reel tapes of varying format to WAV, at 48kHz / 24 bit. Less easy to search than the Soundcloud collection, but more materials available. All free to use with acknowledgement.
Pop Up Archive - This is a service that automatically transcribes and tags a large amount of spoken audio uploaded and made accessible by users. Highlights include the Studs Terkel archive, Society for Baseball Research, Pacifica Radio Archives, and others. The site does not allow users to download any audio, but public collections may host the audio elsewhere, like at archive.org.
University of Iowa, Sounds of Space collection - Collected by University of Iowa instruments on various spacecraft and available for the public to copy/download and use as they wish. The owners ask that a credit line be included that states "courtesy of NASA and the University of Iowa" and, if possible, links to the space audio web site: http://www-pw.physics.uiowa.edu/space-audio/.