If you haven't seen, ah, heard it, you should check it out. Free sounds. For free.
The Freesound Project is a collaborative database of Creative Commons licensed sounds. Freesound focusses only on sound, not songs. This is what sets freesound apart from other splendid libraries like ccMixter. New to this site? Read the What is Freesound page to learn more!
There is some really good stuff on the Freesound Project (some not so great). I stumbled upon a sample of a British WWII air raid siren on there a while back. Lots of field recordings, too.
I'm looking for a file of the background radiation of the universe - Free Sound project doesn't have it. Does anyone know where I can get it?
I took a stab at it and found a simulation. Scroll down and he produces clips.
actually, I was looking for the Cosmic microwave background radiation that was discovered by Arno Penzias and Robert Wilson in 1964, but this simulation may well do nicely for my purposes. Thank you for the reference!