iMovie Normalize clip volume
Audio normalization is a fast way to adjust clip volume to optimal listening levels. Normalization calculates the volume change required to bring the loudest part of the selected audio clip to maximum listening levels without distortion. After this amount is calculated, the volume of the rest of the audio clip is adjusted by the same amount.
To normalize clip volume:
In the Project browser, double-click a clip whose volume you want to normalize.
Click Audio at the top of the inspector that opens.
Click Normalize Clip Volume, and then click Done.