Step 1: Show the audio
Use this button to show the audio waveforms for each of the clips in your project. The audio will appear underneath each of the clips The visual aid will help you judge the start and end of speech.
Step 2: Detach it
Now, from the iMovie menu at the very top of your screen, select the Clip drop-down menu and then choose Detach Audio. It is only possible to do this on a clip-by-clip basis.