Question

With the main knob all the way down on the Studio 192, audio can still be heard through my headphones. Why is this?

Answer 1

Looking at the front of the unit, on the lower left of the Main Knob it states, -80db.

Any signal that is audible to your ears below -80db may still be heard.

The Main Knob delivers 80dB of attenuation and is not a full mute.

For a full mute lower the main output in your software or use mute on your DAW output.