Here is a list of things needed:
1. Sound Effect keychain toy ( please see previous post about sources for those )
2. Opto-resistor with light to dark range around 0k-400k ohms
3. Soldering Iron
4. Knife or flatblade screwdriver
Now, unpackage the keychain, do not insert batteries yet. Pry open the case with a knife or screwdriver. Locate the 150k pitch resistor on the back of the board and unsolder it. Cut the legs of the opto-resistor and arrange as shown in the image to solder it down. With a knife cut a little groove in the top half of the case for the resistor legs to stick through. This makes the case close better. Super glue everything if needed, these keychains are cheap so they will fall apart easy especially if carried in your pocket. Insert the batteries and test. Everything should work. If it doesn’t check all the connections from the speaker and to the board, sometimes they break.
Press a button and change the amount of light to the sensor. The waves will now change pitch as the amount of light is decreased to the sensor. Here are some samples of the unit recorded though a microphone. I suppose the next step is adding a sound-out jack, but that may require some further tinkering and cramming of things inside.