In creating our sound boxes with Jenny, Carmen, and Mouna, we learned both how to construct a circuit from scratch and how to hack an existing circuit. Below is a picture of our breadboard in progress. The breadboards are very handy because they allow us to prototype rapidly, adding and removing components without having to solder.

When opening up the voice recorder key chains, we got to see a PCB, or printed circuit board. A PCB is used when your circuit is fully developed and you know you won't be making any additional changes. They're found inside of every electronics product. Below is an image of the guts of the voice recorder.

This is where the soldering began. We added our own reed switch to the contacts for the playback switch on the circuit board so that the recording could also be activated by a passing magnet. We also removed the connections for the battery and replaced them with wires that connect to the breadboard so that the circuit can be powered by our hand-cranked generator.

And Voila! A two-day intro to physical computing and basic electronics.
If you guys are interested in learning more about this stuff, there are some good basic tutorials on ITP's Physical Computing website, including ones on breadboards and soldering.
The Make magazine blog has many stories about Physical Computing type projects. Also, Instructables is a great site where you can learn how to build things. Instructions for the Personal Power Plant, a project by Jenny, Carmen, and Mouna that's related to the sound boxes, can be seen there.