how to mute sound in imovie

One way to mute sound is to select the clip in the timeline by clicking on it, and then click on the loudspeaker icon in the toolbar at the top of the project screen. That will reveal some audio controls. To the left of the controls there will be another loudspeaker icon that you can click on to mute the clip. To mute the whole project, don't select any specific clip.

Another way is to drag downward the horizontal volume bar in the blue area beneath a video clip or in an added sound track clip. First make sure that you have "Show Wave Forms" enabled via the Settings button located above and to the right of the timeline. Click on the Settings button to get an option box.

-- Rich