Where to find Creative Commons, Public Domain, and other "rights free" or "rights-light" audio material.
Vimeo provides an easy way to quickly find Creative Commons licensed audio tracks to include in your videos. When searchiing the Music Store, toggle on the "License" drop-down menu that allows you to filter on all or a particular type of Creative Commons licenses.
Creative Commons licensed image by Flickr user Colleen Greene.
SoundClouds' advanced search feature allows you to find tracks that you can use for free according to the terms provided in the license. All Creative Commons licenses require at least attribution, which means you must give credit to the original author. (A link to the track page on SoundCloud is probably smart, also).