ummagumma wrote:...and guessing at how the digital pots feed the analog ( I assume? ) mixer
In fact the mixer is not a normal mixer but a array of summing VCA.
The knobs are just doing a drop in voltage.
A multiplexed A/D measures every pot voltage many times a second.
The digital numbers representing the knob position are only used, if there is a certain amount of change since the last loading of a patch. Otherwise the stored virtual knob position number is used. This virtual or real nob position number is than used by a multiplexed D/A to send to the nalaog sound generating board. Also all other informations like MIDI or keyboard notes are D/Aed and sent to the analog board. The analog board got 57 CV controlled parameters. Every continues knob on the front panel is a CV. And there are switched that are controlled by the digital board.
So even the resonance and the individual stages of the envelopes are CV controlled on the Voyager.
So the mixer knobs are in no way connected to the mixer electronics. The mixer just receives more or less steady analog CVs generated from the digital board (by the knob position or the stored patch data). So the mixer knob of VCO 1 is in fact a VCA. And all the mixers channels VCAs are then summed together in a static manner.
All this mixer thing is more or less like on all synths with digital patch memory and real analog sound generating.
You can do a break-out of the signal of the VCA channels of the mixer and change it by some Op-amps to get the right level to mix their signal in a CP3 like mixer (that would be set on a fixed all open position to use patch memory). And then lower the CP3 signal and feed it into the external audio in of the Voyager. I would do this only with the oscillators signal and the noise. The external signal would be untouched on the Voyager's mixer.
Finding the needed points to get the signal would be no problem (but will take some time) if you got a scope and take the needed precautions when working on running opened electronics.