You can use the media browser in iMovie to add songs that you’ve downloaded to the music library on your Mac, as well as songs and other audio files that you've added to your music library. For example, if you want to change the mood of the music over the course of your movie, you can shorten the first song in the timeline, then place a different song after the first song. You can also use more than one song in a project. You can change the length the song like you would any other clip. iMovie automatically places the song at the beginning of the project and automatically fits it to the length of the project. To add the song to your project, tap the Add Audio button next to a song.If you can't find a particular song that you want to use, learn more about why it might not be available. If a song is dimmed, locate the song in the Music app, then tap the Download button for the song to download it to your device.

Tap Audio, tap My Music, then tap a category to browse songs.With your project open in the iMovie timeline, tap the Add Media button.You can add songs from your music library that you’ve downloaded to the Music app on your device, as well as songs and other audio files that have been synced to your device using the Music app on your Mac.

Add music to imovie from youtube