Here's a video of the belt buckle in glorious action:
Step 1: Materials
Super Mario Bros. FX Keychain (available online, $5-10, I got mine at Hot Topic for $10)
On/off switch (I got 2 SPDT switches from Radio Shack for $2.50)
Belt buckle, belt buckle blank (available from craft stores), or wire hanger
Soldering Iron (& flux, solder, etc.)
Hot Glue gun
Dremel or some other tool for cutting the controller board
Wire strippers & scissors
Small phillips scredriver
Drill w/ 1/16" bit
Step 2: Theory
Since both circuits work on the same principle (using conductive buttons to complete circuits), we can use one board to operate the other. All we have to do is solder connections between the hot wire and the out wires for each button. (see pic) Then when a button is pressed on the controller, the keychain is bypassed and the controller button completes the circuit and sends a signal to the microchip on the keychain board, creating a sound effect. All of the buttons work the same way, so any sound can be wired to any button.
The second picture shows the sounds each of the keychain button makes and how I wired them to the controller buttons, FYI.