I am using Audacity 3.0.4-alpha-20210802, as it is the default version in the Linux Mint x64 software manager. I have also tried Audacity 3.3.2 Appimage which has the same problem.
I can set the recording volume for the Jack Audio Connection kit device from SuperCollider on the same computer, but as soon as I start recording the Recording volume will automatically change back to 100% after a few seconds, whether anything is being recorded or not.
At one point (on v3.3.2) the Recording Volume black text on cream info box said “Recording Volume (unavailable; use System Mixer”, however I am unable to alter the recording volume via the system Mixer as the system mixer only alters the mic and line volume, not volume adjustment from another application on the same computer through Jack. I am not aware of any volume contols in Jack.
Although Supercollider has an output volume adjust, it does not reduce the volume of the audio in to Audacity, so I have no way of reducing the recording volume without using Audacity on a separate computer using an audio cable.