As their website states, "The Freesound Project is a collaborative database of Creative Commons licensed sounds. Freesound focusses only on sound, not songs." Read my earlier post to learn how Creative Commons licenses work.
Soundsnap.com is another source "to find and share free sound effects and loops- legally. It is a collection of original sounds made or recorded by its users, and not songs or sound FX found on commercial libraries or sample CD's."
If you're looking for music, check out ccMixter. Again quoting their website, ccMixter is "a community music site featuring remixes licensed under Creative Commons, where you can listen to, sample, mash-up, or interact with music in whatever way you want."
Here's a little rocker from ccMixter...
Remember to check the details of any piece of media with a Creative Commons license. Some prohibit commercial use or require attribution in a manner specified by the author.
And while we're on the subject, to learn more about the sound mixing, Filmsound.org is a great resource.