It’s possible to use a vactrol (optocoupler). I found this schematic:
and I would say: Don’t do that. For one thing, it’s using a CV to drive the LED, and that’s not good for a couple of reasons: CV sources are not meant to supply current, and LEDs don’t respond linearly to voltage; in fact they won’t even turn on until the voltage is high enough. As for using the LDR in series between input and output, what that will do will depend heavily on what your input and output modules are. It might sort of work in certain situations but it’s very overly simplistic.
This is better:
This uses op amps to buffer the audio and CV inputs and to drive the LED for better linearity, and to actively alter the output voltage (instead of just throwing a resistor in series and hoping for the best). It’s designed for a guitar pedal, so there are things you’d change for a synth. Leave out the whole part on the lower left and replace “VREF” with a connection to ground, assuming you’re using a bipolar power supply. I’d replace the 1M resistors with 100k and the 0.22 uF output capacitor with 2.2 uF.
Vactrols are still not greatly linear, and they vary from one to another, and they’re slow, so they’re not ideal for all purposes, but they can be used.