I bought an extensive set of help files for a graphics post production adjustments program.
The narrator turned out to be somewhat high pitched female voice, excited to a rapid rate sometimes.
The files were Mp4 format movies. I needed to adjust at least the speed of video delivery.
Dragging each file onto the editing panel of the current iMovie allowed use of a Slow Motion control.
Set at a custom eighty percent ( 80%) the Mp4 moved at a more pleasant rate for my understanding.
A bonus is the audio frequency also decreased =more bass, less alto.
Not being the file format the iMovie program really wanted to work with, not all program functions worked.
But, for human understanding, the result is better than expected !