I was hoping that all four of the audio generating circuits would shut off when the control voltage was almost at 0v (dot moved to the left of the screen). This is low enough that the tape motors stop turning and the 4046s stop oscillating. Unfortunetely the audio circuits all produced noticable static in this state.

This circuit solves that problem. I feed the control voltage for each of the audio circuits into an LM339 which compares them to 0.5v. When a control voltage is below 0.5v a 4066 analog switch is opened which cuts off the returning audio before reaching the mixer.

The mixer itself is a simple passive design consisting of 100k logarithmic potentiometers and capacitors for AC coupling. Volume control should use logarithmic potentiometers because the human perception of sound volume is approximately logarithmic,

The mixer potentiometers line up with the control button for their respective channel.