All I did was take an existing toy, add a better speaker, bypassing output jack, and in place of the pitch resistor, I put a 10m pot with a 10uF cap parallel, with a cutoff switch and touch points as well. I also did some cosmetic modifications. The result is pretty bizzarre and I have to say, I'm quite pleased with it. Enjoy:
PS: Here is a preview of the official schematic for Digisynth v3. I still have to draw the 16-step sequencer, labels for everything, and the LFO cutoff switches.