Add sound effects in iMovie on iPhone
iMovie comes with dozens of free sound effects that you can add to your movie.
Sound effect clips are colored blue in the timeline. If you add an imported sound effect that’s 1 minute or longer, it behaves the same as background music in the project and is colored green in the timeline. See Add music and soundtracks in iMovie on iPhone.
With your project open, scroll the timeline so that the playhead (the white vertical line) appears over the location where you want the sound effect to begin.
Tap the Add Media button , then tap Audio.
Tap Sound Effects.
To listen to a sound effect before adding it, tap it.
To add a sound effect, tap it, then tap the Add Audio button .