iMovie music copy-write?

If you need to create a soundtrack for your film, that you can use commercially, use Garageband to create the soundtrack with the loops and sound effects from GarageBand. The GarageBand license is less restrictive, and you can install GarageBand 10 for free from the AppStore, if your Mac has been upgraded to macOS 10.2 or a newer system.

See: Using royalty-free loops in GarageBand with commercial work - Apple Support

The GarageBand software license agreement says:

"GarageBand Software. You may use the Apple and third party audio loop content (Audio Content), contained in or otherwise included with the Apple Software, on a royalty-free basis, to create your own original music compositions or audio projects. You may broadcast and/or distribute your own music compositions or audio projects that were created using the Audio Content, however, individual audio loops may not be commercially or otherwise distributed on a standalone basis, nor may they be repackaged in whole or in part as audio samples, sound effects or music beds."

So don't worry, you can make commercial music with GarageBand, you just can't distribute the loops as loops.

Those clips are licensed for personal use, not a commercial use, which would not include the use you describe.