Add audio from a video clip in iMovie on iPhone
When you add a video clip to your project, you can choose to add just the audio.
With your project open, scroll the timeline so that the playhead (the white vertical line) appears where you want to add the audio clip.
Tap the Add Media button , then tap Video.
Tap a category, then browse to a video clip.
Tap the clip, then do any of the following:
Preview a clip before adding it to your project: Tap the Play button .
Add the entire clip’s audio to your movie project: Tap the More Options button , then tap Audio Only.
Trim the clip before you add the audio: Drag the yellow trim handles to select just the portion of the clip you want to add, tap the More Options button , then tap Audio Only.