"Anything with an amplifier should be useful, just find a way to feed the
audio into it. In the old days it was easy, the top of the volume control
would be the place, a coupling capacitor would make sure all was safe.
But now, the volume control may not be a suitable marker, the control may
not be attenuating the audio signal, but varying a DC voltage to control a
stage that is controlling volume."

Or a rotary encoder going to the microprocessor.