iMovie ‘11: Reduce the volume of competing audio clips
You can reduce the volume of audio or video clips that are playing simultaneously, giving priority to the audio you want to hear. For example, you may have added background music and sound effects to your project, but you want the sound effects to dominate. In such a case, you can set all other audio tracks to automatically fade, or “duck,” when the sound effects play.
To duck the volume of competing audio:
In the Project browser, move your pointer over the audio clip (not including background music) or video clip whose audio you want to give priority to, and then choose Audio Adjustments from the Action pop-up menu (looks like a gear) in the lower-left corner.
The track you select here won’t have its volume changed. Rather, the volume will be reduced for any other audio tracks that play simultaneously with the selected clip.
In the inspector that opens, select Ducking.
By default, the volume of ducked tracks is reduced to 15 percent of their original volume.
Drag the ducking slider to the right if you want the ducked tracks to be louder than 15 percent of their original volume, or to the left if you want their volume to be softer than 15 percent of their original volume.
To turn off ducking, double-click the clip, click Audio at the top of the inspector, and then deselect the Ducking checkbox.