What Wikimedia Commons is
Wikimedia Commons is the media repository behind Wikipedia, run by the Wikimedia Foundation. Alongside images it holds a large audio collection: instrument demonstrations, field recordings, bird song, historic speeches and public-domain music.
Best for
Instrument notes and demonstrations, nature and field recordings, spoken word and public-domain recordings.
Licence and what it means for a release
Commons only accepts freely licensed or public-domain files, so non-commercial and no-derivatives licences don't appear. Files are CC0, public domain, CC BY or CC BY-SA. CC BY needs a credit; CC BY-SA would require your track to be released under the same licence, so Sound Server marks it restricted.
How Sound Server searches it
Through the MediaWiki API, restricted to audio files. Each result carries the licence and author from the file's page.