From their site:
The Freesound Project aims to create a huge collaborative database of audio snippets, samples, recordings, bleeps, ... released under the Creative Commons Sampling Plus License. The Freesound Project provides new and interesting ways of accessing these samples, allowing users to
* browse the sounds in new ways using keywords, a "sounds-like" type of browsing and more
* up and download sounds to and from the database, under the same creative commons license
* interact with fellow sound-artists!
We also aim to create an open database of sounds that can also be used for scientific research. Many audio research institutions have trouble finding correctly licensed audio to test their algorithms. Many have voiced this problem, but so far there hasn't been a solution.
How can you help?
If you have audio samples which can be released under the Creative Commons Sampling Plus License please add them! Label them correctly and help us build a huge database.
Note we are not looking for songs, or compositions. We are looking for sounds. Field recordings, kit recordings, generated sounds, ... as long as the sounds are not copyrighted (in a non-compatible way) or if the sounds are made by yourself and you want to release them under the Creative Commons Sampling Plus License.
Freesound is also looking for institutional help with sound files and mirror/server space.