|Singing cards contain speakers inside. These can be removed.|
If you start to take apart the card, you will find that there is a speaker inside, hooked up to some kind of circuit. Chances are, both are coated in a sticky substance, allowing them to stick to the card. Carefully cut them out, allowing a part of the card to serve as their backing, lest you expose the sticky goo. The rest of the card can be, well, discarded. Recycle the paper, and let's get on with the project.
Carefully inspect the circuitry to figure out how the device is triggered. In all of the cards I had collected, there was a thin metal flap. A strip of paper separated the flap from the rest of the circuitry, but when the card was opened, the paper slid aside and allowed the metal to touch. This closed the circuit (I imagine) and started the music.
|With a few household materials, you're ready to go.|
Test out your work. When the door opens, the paper should start to slide away from the musical device. Once the door is wide enough, the paper will be pulled away, allowing the metal to touch and triggering the music.
|When the door opens, the speaker moves away from the paper, triggering the trap.|
There you have it: musical doors. Be sure to hang around so that you can see your target's face when s/he opens it!