Eddie,
that is not a volume control on the back of the PSR2000. It is only an input gain control. It is designed to be adjusted just once and not be touched again unless you change microphones or singers. You adjust this until the green light becomes a flickering red light and then you stop adjusting it. If you want to adjust volume you do this from the vocal harmonizer parameters or from the mixing console settings.
George Kaye
Kaye's Music Scene
Reseda, California