Decrement the volume by 0.1 (or delta) unless it is already 0.
        Returns the new volume.

def volume_down(self, delta=0.1):
        """ Decrement the volume by 0.1 (or delta) unless it is already 0.
        Returns the new volume.
        """
        if delta <= 0:
            raise ValueError(
                "volume delta must be greater than zero, not {}".format(delta))
        return self.set_volume(self.status.volume_level - delta)