While in-game, go to the clipboard and select OPTIONS -> CONTROLS -> MAP CONTROLLERS from the clipboard menu. You can "double-click" the control on your throttle that you want the function mapped to. This should select it and put you in edit mode so you can map the "Voice Transmit Range" function to that button. You can do this for any button on any device (Joystick or Throttle) and map any function you want to any button you want.