The IA was founded to provide a cultural "Internet library"; a sort of Library of Alexandria for the modern age (hopefully without the barbarous hordes burning and pillaging it).
The Archive has an enormous list of audio, video, graphic, and text files on nearly every subject. Looking for an MP3 of a 1927 blues singer? You'll probably find it.
There are loads of files that have either Creative Commons licensing, or are Public Domain. Which means that you can use the files as long as they're for non-commercial use and are properly attributed.
So, I'm downloading archive video and background music for my public access documentary. Check out the site; you'll probably be amazed at the amount of interesting stuff there.