Alternative to potentiometer for setting guitar volume
The vactrol works as a gain-setting element for the op-amp. Gain will be 1+R15/R(vactrol). The more the LED light, the lower R(vactrol), and the higher the gain.

Another way to do this is to use a voltage-controlled FET as an attennuator. Here is an EDN article describing how to do that: https://www.edn.com/a-guide-to-using-fets-for-voltage-controlled-circuits-part-2/