You should always verify that the work is actually under a CC license when conducting a search on any website. If you are in doubt you should contact the copyright holder directly, or try to contact the site where you found the content.
This library contains recordings ranging from alternative news programming, to Grateful Dead concerts, to Old Time Radio and most are downloadable, but all materials on the site are free to use/share.
The FMA is a collection of high-quality, legal audio material. All material on the site contains a Creative Commons license so is available to use for free.
A social sound platform for creating and sharing music and sounds. Many SoundCloud songs and sounds are licensed with Creative Commons. To search for items that allow reuse, after a search, filter the search results by "To share", "To modify", etc.
