Here is a schematic promised for a very long time. 4 Oscillator circuit based on the 4093 Quad Shmitt Trigger IC. I have used identical circuits in the GameBoy Theremin Synth and the Atari Joystick Synth. There is not much to creating an oscillator. In this case 1uF electrolytic capacitors was placed between the input pin and the ground. Also the Optical resistor from the output pin of each individual gate back to the same input pin as the cap, this creates the feedback needed for oscillation. For this project a 0k to 200k photoresistor worked fairly well, 0 to 15k would work but not as many low frequencies, more upper range, this can be countered with a 10uf cap of needed. The signals from each oscillator are mixed by diodes in the direction of the output. If this is not there and the outputs are just soldered together, the circuit will not work. This solution has huge limitations and drawbacks, because certain frequencies are louder then others. An optimal solution is to use a Quad Op-amp to do the mixing. This IC will run on a 9Volt battery with all the ground connections made to the Vss Pin 7. The +9 goes to the Vdd pin 14. The schematic assumes proper power applied to the IC. The free input pins can be used for touch controls and or inputs from the other oscillators. Once each individual oscillator is built, various output to input combinations can be tried. Good luck and let me know if I messed something up.