Add audio from a video clip in iMovie on iPhone
When you add a video clip to your movie project, you can choose to add just the audio.
In the iMovie app on your iPhone, open a movie project.
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 .
You can also swipe left and right over a clip to manually preview the video, which can help you identify exactly which part of the clip you want to use.
Note: If you use iCloud Photos, some media may be stored in iCloud but not on your device. If a Download button appears when you tap a clip, tap the button to download the clip to your device.
Add the entire clip’s audio to your movie project: Tap the More 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 button , then tap Audio Only.