I worked a bit on the audio modulation circuit. It will make things move on the screen with the audio input signal.
At first I though about controlling the analog switch with three microcontroller pins . Although it'd be a simpler solution, both circuits are not on the same board which will create a need for another ribbon cable. I wanted a bit more fun with digital logic and came up with the switch controler on the right.
Value for the filters will certainly be variable with trimmers.
The input buffer will be able to accommodate low signal (headphone output of mp3 player), line level signal and modular level signal (very hot 10Vpeak-peak). R1 will be made variable to attenuate and control the depth of modulation.
The output will probably be clipped at 1V to not exceed the standard of the synth.