I found audio and video files of my synthesizer of 2009. The circuit is easy built, ready for DIY. Minimal required is a quad NAND Schmitt trigger logic IC “4093” to get RC oscillators with control input.
The oscillation occurs due to the bistable characteristics of the Schmitt trigger input. It detects a high signal only after an upper voltage level is reached and low beyond the lower trigger level. The hysteresis is about Vcc/5. The frequency can be calculated easily when considering the exponential charge curve: oscfreq.pdf
The circuit shows one RC-oscillator and an optional amplifier. With jumpers you can select the output signal and let oscillators enable/disable each outer. If the enable input is unconnected a resistor maintains the high state of the input.