def apply(self, clip: Clip) -> Clip:
        """Apply the effect to the clip."""
        if self.start_time is None and self.end_time is None:
            return clip.transform(
                lambda get_frame, t: self.factor * get_frame(t),
                keep_duration=True,
            )

        return clip.transform(
            self._multiply_volume_in_range(
                self.factor,
                clip.start if self.start_time is None else self.start_time,
                clip.end if self.end_time is None else self.end_time,
                clip.nchannels,
            ),
            keep_duration=True,
        )
Apply the effect to the clip.
moviepy/audio/fx/MultiplyVolume.py