Very few sound recordings are in the public domain in the United States; in fact, most sound recordings will not enter the public domain until February 2067! However, there are many sound recordings that have been licensed with Creative Commons. See the Creative Commons page for resources.
Large and growing collection of spoken word and musical works mostly found in the public domain.
MP3 recordings of public domain music.
NASA hosts several image collections, including photographs taken by astronauts and images from the Hubble Space Telescope. As works created by the federal government, they are in the public domain.
Public Domain works are not protected by copyright law and, therefore, they are freely available for everyone to use. There are several ways in which a work passes into the public domain:
What this means for your academic projects is that you can use as much of the work as you would like to support your instruction, research, publication, creative work, etc. without needing permission from the original copyright owner.