Hi David!
I have the two relevant parts of the MIDI Guitar interface marked by a red box and a blue box here. Red is for any synth/virtual instrument and effects to go with that, whereas the blue box is the Audio part of the signal chain. The MIx knob sets the balance between the two.
Now, if you have nothing open in the blue box (like in the picture) you shouldn’t have any guitar audio coming through however you set the mix knob.
If you do it is usually a “direct monitoring” setting on your audio interface that needs to be addressed.
Check these discussions out, and see if there is something in those that might help:
