Step 1: What You Need...
1. A calculator
2. A device that uses a recording module. In my case I found a cheap automotive memo thingy that sticks to the windshield and allows you to record up to 5 different messages and play them all back. You could use a regular single sound module like one from Radio Shack.
NOTE: You will have to find a calculator that is thick eoungh to hold the module inside it's case.
Step 2: Prepare the Recording Module
Before you do anything else, record what you want the calculator to "say" on the module. I actually ran a jumper from the microphone input to a mini plug that plugged into my audio card on my PC and then I recorded some .wav sound effects.
Then disassemble the device to get to the recording module. You can clip the microphone connections or leave them depending on how much room you have inside the calculator.
Step 3: Prepare the Calculator
Take apart the calculator and decide what key you want the sounds to play from. I chose the EQUAL sign key because that always has to be pressed. You could choose a number key that would play the sounds only when that number was pressed or the minus key...your choice.
Step 4: Make Your Connections
Run two small wires from each side of the contact on the calculator key you decide to use. Then run those two wires to the playback contacts on the module. Run the two power wires from the module to the positive and negative ends of the calculator battery terminal. When the calculator key is pressed, it will complete the circuit for that key and also the playback button.
Step 5: Close It Up
Close up the calculator and make sure no wires are sticking out. Re-insert the battery and your calculator prank is ready to use! Each time the "=" sign is pressed, the module will playback the 5 recorded sounds and then repeat. Have fun!