If you're looking for audio files that you can add to your podcast, there are a few options. Two free, popular options are Creative Commons content and Public Domain content.
Creative Commons created a series of copyright licenses that allows content licensed under Creative Commons licenses to be used for free so long as attribution, or credit, is given to the creator. Different types of uses are allowed under each license, such as allowing changes to be made to the content or not allowing commercial uses. When you find something licensed under Creative Commons, it will list the type of Creative Commons license it's licensed under. For a complete list of the Creative Commons licenses or to see examples of attribution, please visit CreativeCommons.org.
Works that are in the Public Domain are no longer under copyright protection, or were never under it. This type of content can be used freely without permission and without restrictions, and attribution isn't required.
Below you'll find some resources where you can find Creative Commons and Public Domain content.
These only have Creative Commons or Public Domain content:
These have Creative Commons, Public Domain, and copyrighted content (each item should list which category and copyright license it falls under):