It is very easy to add a volume pot to the speaker output of a circuit bent toy. This control cannot make the toy louder, but it can set the output to a softer volume as desired. To achieve a voltage output curve that is somewhat comparable to the way that humans perceive volume, it is recommended to use what is termed a 'logarithmic' or 'type a' potentiometer.
1. Consider a potentiometer, with its legs numbered from one to three. It doesn't matter which outer leg is one and which is three, so long as the middle one is two.
2. Snip the connection between the speaker and the toy at approximately the halfway point. Then strip back the wire of each of the four ends and tin them with a small amount of solder.
3. Solder the two wires from the speaker to legs two and three of the potentiometer.
4. Solder one of the wires from the output of the circuit to leg one of the potentiometer. Solder the other wire to leg three of the potentiometer.