There is a cool little feature in iMovie called “Ducking.” This is a great feature that automatically lowers the background music so you can hear your subject talking.
The trick to remember is that you want to do this to the clip that you WANT to hear and not the surrounding clips. In iMovie, simply select the clip that you want to hear (e.g. the subject talking). Then choose the audio adjust button. Check the box for Ducking. You can then slide the percentage bar to adjust how much the other clips’ volume is adjusted. That’s all there is too it. Now when you playback your clip, the background audio will automatically lower when it reaches the subject talking clip.