code: def on_click(self, event):
          
          button = event["button"]
          # volume up
          if button == self.button_up:
              self.backend.volume_up(self.volume_delta)
          # volume down
          elif button == self.button_down:
              self.backend.volume_down(self.volume_delta)
          # toggle mute
          elif button == self.button_mute:
              self.backend.toggle_mute()
docs: Volume up/down and toggle mute.