iMovie
Change audio volume in iMovie on Mac
As you adjust the volume of a clip in iMovie, its audio waveform changes shape and color to reflect your adjustments. Make sure that the peak sections of the waveform don’t appear yellow, which indicates distortion, or red, which indicates clipping (severe distortion). If you see either red or yellow in your audio waveform, lower the volume until the entire waveform is green. If only part of the waveform is red or yellow and the rest is green, you can adjust the volume of that part of the waveform.
If waveforms don’t appear in the timeline, click the Settings button in the upper-right corner of the timeline and select the Show Waveforms checkbox.