There are several ways to extract just the audio from a video clip in a project or Event and add it to another clip in a project. The extracted audio clip behaves just like any other audio clip in iMovie. You can move it within the project, trim it, apply an audio effect to it, and so on.
To add just audio from one video clip to another:
In the Project browser, select a video clip or a frame range, and then choose Clip > Detach Audio.
The Clip menu appears in a light gray bar across the top of your computer screen.
The now-detached audio track appears as a purple bar below the video clip.
Drag the purple bar and position it below any video clip in your project.
When you move the audio clip, the video clip where it originated no longer has an audio track.
To add just audio from an Event clip to a project clip:
Open the project you want to add audio to so that it’s showing in the Project browser.
In the Event browser, select a video clip or a frame range that has the audio you want to use in your project, and drag it directly over a clip in your project.
Release the mouse button when you see a green Add (+) symbol.
In the menu that appears, choose Audio Only.
This action copies the audio in the Event clip to the selected project clip; it doesn’t remove the audio from the Event clip.
If you see several more options in the menu, it means Advanced Tools are turned on.
You can also add just the audio from an Event clip by holding down the Command and Shift keys as you drag the clip to your project.
If you add audio to a clip that already has sound, you can reduce the volume of competing clips so that just one clip’s audio has prominence. Go to this topic to learn how: Reduce the volume of competing audio clips.