Audacity: open source digital audio editor
Audacity Manual: a comprehensive guide to using Audacity.
The Free Sound Project: a massive library of downloadable and sharable sounds to use in audio projects.
Creative Commons Audio: another place to look for sound effects and music (with appropriate licenses that indicate how people should use and/or cite borrowed sounds).
Partners in Rhyme: contains public domain and royalty free sound effects and music.
Sound Bible: more free sounds
Jamendo: free music downloads