Separating the Audio Track from the Video
When you drag a clip from the Shelf into the Viewer, the video and audio appear as a single track in the Viewer. But you can extract the audio from the video, creating separate tracks for each.
You would do this if you want to use the video in a clip but with some different audio, such as voice-over narration describing the video.
Or you would do this if you just want the audio from a clip and don't need the video.
Thus if you want to add voice-over narration to a video clip, you could record the narration using the microphone in a video camera, with the lens cap on. Then capture the narration clip in iMovie, move the clip to the Timeline, and extract the audio. Delete the blank video track and drag the audio track to accompany some other video in the Timeline.
To separate the audio from the video, select the clip in the Viewer.
Then in the menu at the top select Advanced...Extract Audio.
In the Viewer Timeline display, you'll see that the audio is separated out and appears as a distinct audio track just below the video track (if your audio was recorded in stereo, then it will appear as two tracks of audio on the Timeline).
You now can move the audio track separately from the video by clicking on the audio and dragging it to the left or right along the Timeline.
You also can edit the audio track to eliminate some of it. Click on the audio clip and move the Playhead to a point in the audio track. Then in the menu at the top select Edit...Split Selected Audio Clip at Playhead. Now click on the segment of audio you don't want and press the Delete key on your keyboard to eliminate it. Repeat the process to edit other segments of the audio.
And you can change the audio levels on the clip just as you would if it were still part of the video clip, by manipulating the purple audio level line.
Or you can highlight the entire audio track for the clip and press the Delete key on your keyboard to eliminate the entire audio portion of the clip (or similarly you can highlight the video track and use the Delete key to eliminate it and only preserve the audio.)