I think the original 'hard drive speaker' idea came from Afrotech a couple years ago:
Step 1: Open the Hard Drive
You'll end up seeing something like this.
Use a hacksaw to cut as far as you can between the hard drive arms and the platters. Then for the final bit of metal under the arms, use a dremel from the backside of the case. Don't worry about cutting through the flexible cable going to the arms, you should be able to see where the arm voice coil (the coil of copper wire between the two magnets at the arms base) is soldered to the which you're cutting.