You Will Need:
-obnoxiously loud electronic toy
***EDIT: I am NOT an electrician. I have no idea if this is safe to do. A couple of people have commented that placing cotton balls inside an electronic device could pose a fire hazard. DO THIS AT YOUR OWN RISK.
Step 1: Remove The Batteries
Step 2: Open The Toy
*** EDIT: be very careful while dismantling/reassembling your toy. Do not pinch any wires. Put everything back exactly how it was. Take pictures along the way so you remember where everything goes.
Step 3: Locate The Speaker
Step 4: Stuffing The Speaker
***EDIT: some people have commented that cotton balls are flammable and putting them inside an electronic device could pose a fire hazard. Do this at your own risk.