Apple just added lots of new songs to iMovie’s free music library. You can pick music from many genres and movie themes. There even are a couple of Hans Zimmer songs in there.
A nice feature is that the music adapts to the length of your movie. Try it out, it's pretty cool.
Have you added background music to any of your videos?
(Unfortunately screen recording in iOS doesn’t record stereo sound. That’s why the video sound isn’t perfect. In reality, the music in iMovie has top-class sound quality.)