Possible Modifications to the Circuit
While we've created a useable circuit, it would be nice to modify the circuit to be more fun. The original circuit had a complex input system (a dual-optical diode that was a little overblown for our needs); we can accomplish the same thing by replacing our potentiometer with a photoresistor. In addition to covering up the resistor to change the pitch, you can also use a flashlight to excite the photoresistor in an interesting way.
We can also change the circuit to create events by placing a button between the speaker jack and the system. This allows us to manually turn the sound on and off.
Finally, we can use something like a force-sensing resistor to give us physical control of the output volume. FSR's, photoresistors and buttons all act as ways of changing the electrical flow from some positive voltage to some negative voltage. Many other sensors perform similar functions; you could just as easily use bend sensors, sonar detectors and other tools for interesting results.
Another modification technique is to try replacing resistors and capacitors with components of different values. Sometimes the sound will change, sometimes the circuit will stop working. Sometimes, something will blow up! In any case, experimenting with this is part of the experiential nature of an artistic view of electronics.
Finally, you can replace the speaker with an audio jack, and you can pipe the sound from your circuit into a computer or amplified speaker. However, you should be careful before you do this: measuring the output voltage levels (which should be under approx 1.5 volts) will allow you to do this without harming the machine.
That's just a start - I hope this gives you some excitement about exploring on your own!