I'm working on an app that needed just the right sound effect for one of the pages. I whipped up a quick flex app to play the sound easily enough, but got stuck trying to figure out a source for sound effects. Any search that included the word mp3 seemed to return back junk/dead links.
I'm using this sound for development purposes, so I didn't investigate the license too thoroughly. Though their request of not selling or posting the audio file was easy enough to comply with.
If you need a sound effect, soundjay seems like the right place to start.