Inside the chest cavity there is a miniature circuit board with a rubber pushbutton switch. Attached to this circuit board is a small speaker connected via two short wires and a set of three button cell batteries. This entire assembly is held in place by two white plastic posts. Use your handy narrow blade straight screwdriver to gently pry the circuit board up and off of these two plastic posts.
Note: the three button cell batteries could "spring" out of the circuit board's battery clips. Don't lose these batteries. Keep them together with a short piece of transparent tape.