There are a number of free sound effects resources of public domain or free content sound recordings appropriate for Wikipedia use available online, and as well as in other contexts.
All files should be converted to ogg, Wikipedia's patent-free format of choice.
American Folklife Center – a very diverse collection of folk music and other ethnographic/documentary materials by the U.S. government. As U.S. government works, these are all available under the public domain.
Cylinder Preservation and Digitization Project - an academic project preserving commercial recordings from the age of the phonograph cylinder. Both mp3 files and wav files are provided for all titles; the mp3 files are placed under a non-commercial license, so only conversions of the wav files are appropriate for Wikipedia use.