How to use the pydub.utils.audioop.mul function in pydub

def apply_gain_stereo(seg, left_gain=0.0, right_gain=0.0):
    """
    left_gain - amount of gain to apply to the left channel (in dB)
    right_gain - amount of gain to apply to the right channel (in dB)
    
    note: mono audio segments will be converted to stereo
    """
    if seg.channels == 1:
        left = right = seg
    elif seg.channels == 2:
        left, right = seg.split_to_mono()
    
    l_mult_factor = db_to_float(left_gain)
    r_mult_factor = db_to_float(right_gain)
    
    left_data = audioop.mul(left._data, left.sample_width, l_mult_factor)
    left_data = audioop.tostereo(left_data, left.sample_width, 1, 0)
    
    right_data = audioop.mul(right._data, right.sample_width, r_mult_factor)
    right_data = audioop.tostereo(right_data, right.sample_width, 0, 1)
    
    output = audioop.add(left_data, right_data, seg.sample_width)
    
    return seg._spawn(data=output,
                overrides={'channels': 2,
                           'frame_width': 2 * seg.sample_width})

if duration:
            if start is not None:
                end = start + duration
            elif end is not None:
                start = end - duration
        else:
            duration = end - start

        from_power = db_to_float(from_gain)

        output = []

        # original data - up until the crossfade portion, as is
        before_fade = self[:start]._data
        if from_gain != 0:
            before_fade = audioop.mul(before_fade,
                                      self.sample_width,
                                      from_power)
        output.append(before_fade)

        gain_delta = db_to_float(to_gain) - from_power

        # fades longer than 100ms can use coarse fading (one gain step per ms),
        # shorter fades will have audible clicks so they use precise fading
        # (one gain step per sample)
        if duration > 100:
            scale_step = gain_delta / duration

            for i in range(duration):
                volume_change = from_power + (scale_step * i)
                chunk = self[start + i]
                chunk = audioop.mul(chunk._data,

start_frame = self.frame_count(ms=start)
            end_frame = self.frame_count(ms=end)
            fade_frames = end_frame - start_frame
            scale_step = gain_delta / fade_frames

            for i in range(int(fade_frames)):
                volume_change = from_power + (scale_step * i)
                sample = self.get_frame(int(start_frame + i))
                sample = audioop.mul(sample, self.sample_width, volume_change)

                output.append(sample)

        # original data after the crossfade portion, at the new volume
        after_fade = self[end:]._data
        if to_gain != 0:
            after_fade = audioop.mul(after_fade,
                                     self.sample_width,
                                     db_to_float(to_gain))
        output.append(after_fade)

        return self._spawn(data=output)

chunk = self[start + i]
                chunk = audioop.mul(chunk._data,
                                    self.sample_width,
                                    volume_change)

                output.append(chunk)
        else:
            start_frame = self.frame_count(ms=start)
            end_frame = self.frame_count(ms=end)
            fade_frames = end_frame - start_frame
            scale_step = gain_delta / fade_frames

            for i in range(int(fade_frames)):
                volume_change = from_power + (scale_step * i)
                sample = self.get_frame(int(start_frame + i))
                sample = audioop.mul(sample, self.sample_width, volume_change)

                output.append(sample)

        # original data after the crossfade portion, at the new volume
        after_fade = self[end:]._data
        if to_gain != 0:
            after_fade = audioop.mul(after_fade,
                                     self.sample_width,
                                     db_to_float(to_gain))
        output.append(after_fade)

        return self._spawn(data=output)

self.sample_width,
                                      from_power)
        output.append(before_fade)

        gain_delta = db_to_float(to_gain) - from_power

        # fades longer than 100ms can use coarse fading (one gain step per ms),
        # shorter fades will have audible clicks so they use precise fading
        # (one gain step per sample)
        if duration > 100:
            scale_step = gain_delta / duration

            for i in range(duration):
                volume_change = from_power + (scale_step * i)
                chunk = self[start + i]
                chunk = audioop.mul(chunk._data,
                                    self.sample_width,
                                    volume_change)

                output.append(chunk)
        else:
            start_frame = self.frame_count(ms=start)
            end_frame = self.frame_count(ms=end)
            fade_frames = end_frame - start_frame
            scale_step = gain_delta / fade_frames

            for i in range(int(fade_frames)):
                volume_change = from_power + (scale_step * i)
                sample = self.get_frame(int(start_frame + i))
                sample = audioop.mul(sample, self.sample_width, volume_change)

                output.append(sample)

chunk = self[start + i]
                chunk = audioop.mul(chunk._data,
                                    self.sample_width,
                                    volume_change)

                output.append(chunk)
        else:
            start_frame = self.frame_count(ms=start)
            end_frame = self.frame_count(ms=end)
            fade_frames = end_frame - start_frame
            scale_step = gain_delta / fade_frames

            for i in range(int(fade_frames)):
                volume_change = from_power + (scale_step * i)
                sample = self.get_frame(int(start_frame + i))
                sample = audioop.mul(sample, self.sample_width, volume_change)

                output.append(sample)

        # original data after the crossfade portion, at the new volume
        after_fade = self[end:]._data
        if to_gain != 0:
            after_fade = audioop.mul(after_fade,
                                     self.sample_width,
                                     db_to_float(to_gain))
        output.append(after_fade)

        return self._spawn(data=output)