When you turn on theme music in project settings, the music dynamically adjusts to fit the length of your project.
In the iMovie app
on your iPad, open a movie project.
Tap the Project Settings button
.
Tap to turn Theme Music on or off, then tap outside the project settings to close them.
You can also add theme music to your project separately, in the same way you add a soundtrack. See Add music and soundtracks in iMovie on iPad.