Please check the type of license and its usage terms before downloading and using free clips.
Audio library YouTube offers a growing library of music for use in video making. This is a very simple download process and music can be searched by genre, mood, instrument and duration.
ccMixter: a community music remixing site featuring remixes and samples licensed under Creative Commons licenses. You are free to download and sample from music on this site and share the results. Some songs might have certain restrictions, depending on their specific licenses. Each submission is marked clearly with the license that applies to it.
Five Great Sources For Free-To-Use Audio Clips & Sound Effects provides five different options for finding sound effects. See the comments below for even more.
Free Sound Effects: provides free sound effects including animals, people, vehicles, weather, and special effects. The Mixcraft software can be downloaded to mix sound effects.
Freesound.org – Sounds: a collaborative database of audio snippets, samples, recordings, bleeps, released under Creative Commons licenses that allow their reuse.
PacDV Free Sound Effects provides royalty free sound effects and a royalty free music collection.
Royalty free sound effects has free sound effects.
Sound Effect Libraries also has free sound effects.
The Free Music Archive is an interactive library of high-quality, legal audio downloads.
Tips for using sound
Simple sound tips from Moviestorm offers some useful ideas for improving the sound in your movie.
Audacity is free, open source, cross-platform software for recording and editing sounds. Audacity is available for Windows, Mac, GNU/Linux and other operating systems.