If you mean this:
http://sonyclassical.com/sounds/47668_01.wav
The opera is called "Carmina Burana", the actual song is called "Oh Fortuna" and it is available on Sony Classical CDs. You can try iTunes to download it, but it may not be there as Sony have their own (somewhat more restrictive) download store..
http://sonyclassical.com/releases/47668.htm