Use analogue joystick(s) to control the V-Theremin.

Button 1 = Toggles Mute
Button 2 = Toggles between 1 and 2 analog sticks for control.
Button 4 (on either stick) allows you to crudely play melodies without moving up and down to control volume.

When using 1 joystick :

Joy 1 Y Axis = Volume
Joy 1 X Axis = Pitch

When using 2 joysticks :

Joy 1 Y Axis = Volume                                                                                                
Joy 2 X Axis = Pitch