Mini Synth demo (that one on 'GGG' originally from Funway Into Electronics 3)

[anotherjim]

Had another thought about the switches. The original idea must have switches with contacts for all 3 throws. A DPDT on-off-on won't do it.
But looking at the connection choices...
4046 pin 3 connects from only 2 places. It's pin 4 (VCO out) or a flip-flop Q output.
Audio output comes from only 2 places. 4046 pin4 or a Q output.
... so you could use a pair of SPDT switches. 2 of the 4 possible combinations will be the same, but you still get the original 3 selections.

There are a few more sound sources that are being unused.
The 4046 phase comparator output on pin 2 will have a signal. It won't always be a square wave unlike all the others and is more like an XOR frequency doubler.
The pin1 phase detector pulses have a glitchy character of their own.
The VCO CV input can be used for a VCF CV source. The chip has a source follower buffer from pin10 to tap the CV from.
The Envelope for the VCA transistor could also be used as a VCF CV, although you may need to buffer/amplify that.