When making a movie, you often want to have audio from one clip play during video from another clip, or make a clip silent, or otherwise manipulate audio and video seperately. iMovie makes this process simple, but only if you know where to look for the controls, and how to manipulate the results.
To extract the audio from a clip, first place the clip in your timeline and click on it so it is highlighted in yellow. Then, right click (or control-click on a Mac) to open the dropdown menu, and select “detach audio.”
This will create a new item in your timeline: a thin purple rectangle which represents your audio track.
This is also how your audio will appear if you import an audio file. Once you have it separate, you can drag the audio file anywhere you want it in the timeline. Here, I’ve moved it later in the movie, so it starts in one clip and continues playing over the next:
Notice the little purple triangle aligned with the red line. This allows you to place your audio with more precision. Moving your cursor over the audio file will play the file in time with your movements. If you want to sync a particular part of your audio file with a particular frame in a video clip, simply move your cursor over the file until the red line is aligned with the desired part of the audio; then click the audio file and drag it along your timeline. The purple triangle will mark the audio moment that interests you, and it will remain aligned with the red line; when the previewer on the right displays the frame that you want synced with the selected audio moment, just release the file and you’re done!
It is also possible to cut off part of the audio by clicking either the left or the right end of the rectangle and dragging the edge to its new position, as you do when selecting part of a clip. If there is more audio associated with the clip, you can also extend the audio file the same way. And, of course, you can delete the audio entirely. The same holds true for audio imported from other sources. Audio files can be stacked on top of each other, too. iMovie is deceptively simple, but it can be a powerful tool if you know where to find the controls.