Dr. Richard Boulanger and colleagues have created an 8.5 GB audio library of
samples and sounds which has been licensed under the Creative Commons
license. It is available at: http://wiki.laptop.org/go/Sound_samples
It is aimed primarily at participants of the One Laptop per Child / XO
laptop project but its a great source of public domain audio material as you
can pretty much use them for any purpose once you attribute the source of
the materials.