Hand tools and fabrication machines
The most recent cake I hacked was for the University of Colorado Boulder's Blow Things Up Lab. The cake screams "ow!" every time it was cut.
The brain of this cake was an Arduino derivative called a Makey Makey that turns whatever material you're working with (cake) into a keyboard input. Plug the Makey Makey into your computer via USB.
Connect one alligator clip to a metal butter knife and the spacebar pad on the Makey Makey. Connect another alligator clip to the edge of the cake's frosting and the ground pad (called "Earth") on the Makey Makey.
I deviated from my usual code editor and wrote the code in Scratch because it allowed me to program the Makey Makey in literally one minute. Scratch has a great sound library that already had a "scream" track. Here is my super simple code:
To create this code in Scratch, go to Scripts > Events and drag the icon that says "when space key pressed" to the right sidebar. Then click Sound, followed by the speaker icon. The speaker icon will pull up the sound library. I clicked on "scream-female", but you could use any sound. Go back to Scripts > Sound and drag the icon that says "play sound scream-female" to the right sidebar. Finally go to Scripts > Control and grab the icon that says "wait 1 secs" and drag is to the right sidebar. Stack up the icons on top of each other so it looks like the above image.
That's it! Two connections to the Makey Makey and a laptop is all you need to add sound effects to your cake. When the circuit is completed between the frosting and the knife, the cake will scream. Make sure to wash your alligator clips before and after food use! Alternatively you could probably use aluminum foil between the cake and alligator clip if you're concerned about food safety.