It sounds to me like you're looking for one of the speakers in this set, but unfortunately it's out of stock...
http://www.xoxide.com/hspc-atx-control-kit.html
You can usually just pull one out of any other case, so if you see an old broken computer on the side of the road then just yank one out... or if you have another computer you can borrow it from, do that...
Does your computer still do this when trying to boot from any other device (i.e. a Windows setup disk, Bart PE or Knoppix CD)?