Sound is not ancillary to picture. In many cases, sound is more important than picture in giving the audience a feeling of completion and quality.
You use the advanced feature Extract Audio to separate video material into its component picture and sound elements.
You can unlock picture and sound elements before moving them around by using Unlock Audio Clip.
Drag sound clips to any track. If you want to hear different sounds simultaneously, they need to be in different tracks.
Use the microphone in iMovie's Audio pane to record narration right into the timeline.