Below is a list of sites containing free audio files. Please note that not all the files on these websites are free. Notably non-commercial (NC) and no derivatives (ND) clauses are common, but not accepted on Commons and will lead to your file being deleted.
ccMixter.org - Music under Creative Commons Licenses. Important: Only music under Attribution, Attribution-Share alike and CC0 can be used. Note: Be careful some users may not license their work properly, because they remix work under other non compatible licenses or they may play copyrighted work. Creative Commons sampling licenses are not free enough to upload to Wikimedia Commons.
Raita is a collection of digitized early Finnish sound recordings provided by the Library of the National Library of Finland. All music files are under public domain (over 50 year old recordings of public domain music).
freemusicarchive - published under various CC licenses, just passed 100,000 tracks in 2016; more information here
LibriVox - Project to record books in the public domain.
Musopen provides music in the public domain by major composers: music recordings, sheet music and a music textbook. Completed in 2012. See also: https://archive.org/details/musopen.
The Mutopia Project. Free sheet music for everyone. "Most of our music is distributed under Creative Commons licenses. Each piece clearly lists what license it is distributed under."
Opsound - Sounds and music under "Creative Commons Share Alike license".
Public Domain Project is a growing collection of digitized gramophone records (78 rpm), all of them nowadays belonging to the public domain.
YouTube audio library is a collection offered by YouTube to their content creators. Only the "Attribution required" songs are free. You can select them from the "Attribution" drop-down on the top of the list.
Open Game Art - Media repository for software / game projects. Contains many kinds of media, including sound effects, under various free licenses.
Kenney.nl - Website offering assets for video game projects under a public domain (CC0) license. Contains many kinds of media, including sound effects.
