In the volume menu set the volume you would like on the max setting and then on the volume control assign the flight phase (FP1, FP2 etc) then when you select that flight phase the vario tones are active as the flight phase switches between silent and max volume.
Note if the effect is reversed, i.e. vario tones don't play in the selected flight phase then in the volume menu highlight the control and press rev/clr to reverse the action.
This how I use the vario on several of my gliders.
