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.
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.