Follow these steps to add music to a video on iPhone:
Before we add music, you have to create a Project in iMovie.
Start by opening iMovie. Tap the + button to make a new project.
Then Tap Movie.
Selectthe video you want to add music to.
Tap Create Movie at the bottom of the screen.
This will bring you to your project timeline, the main hub of editing a video. Now we can add music.
1. To add music tap the + button
2. Tap Audio.You can select built in soundtrack music and sound effects included with iMovie, songs you’ve downloaded to your device with the Music app, or your own custom songs created in GarageBand.
Tap the audio track you want to use. You might be prompted to download the song to your device before you can preview it. To add the song to your project tap the + button with a circle around it.
3. The song will be added at the bottom of the project timeline, starting at the beginning of your project. By tapping on the song (shown in green) it opens the song editing menu, where you can adjust the length, split the track, change volume, and many other options.
When you are happy with your edits, tap Done and export your file.