The audio output is a stereo socket, but only the left channel (tip of the plug) carries the audio signal. The right channel (ring of the plug) has the GATE/TRIGGER signal for synthesizers. If you mix both channels accidentally together, the GATE pulse will be audible as a click when your left hand goes through a defined height.
To prevent this effect, use a splitter cable and feed only the left channel into the amplifier.
More details on the OTv4 website: https://www.gaudi.ch/OpenTheremin/index.php/opentheremin-v4/control-voltage-gate