Step 1: What You Need...
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
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.