Microkits Theremin: reversing the Volume

Hi

I got the theremin from Microkits and it works. As a result, I would like to use data from the Theremin on my Nano using a DAC. But first I have to reverse the volume: at the moment the volume increases when you move your hand away from the antenna. This means that the theremin squeaks constantly.
Maybe there is someone here who can help me to implement this or tell me if this is even possible.

I think easy way to start is to draw this schematic in LTSpice (sensor part), and then play with it.
BTW, RC sensor seems to have awful sensitivity. What is a playable distance for pitch antenna?