For information purposes, Paragraph 2G of the iMovie license provides in part:
"The Apple Software contains media content including but not limited to
graphics, audio clips, video clips and/or templates (“Media Content”). This
Media Content is proprietary to Apple and/or its licensors, and is protected by
applicable intellectual property and other laws, including but not limited to
copyright. Except as otherwise provided, all Media Content included in the
Apple Software may be used on a royalty-free basis in your own video projects
but may not be distributed on a standalone basis (and audio Media Content may
only be used for your own personal, non-commercial use)."
Open iMovie and click on iMovie/About iMovie/License agreement to confirm the language for yourself, as sometimes the language can change with new iterations of iMovie.
So it appears from the above quoted language that iMovie music would be copyrighted and proprietary. As you can see from the license language quoted above, "Royalty-free" doesn't necessarily mean copyright free. Also, if you might be monetizing the sound clip that may run afoul of the "personal, non-commercial use" restriction of the license.
As Apple advises elsewhere in the license you would need to consult an attorney for a definitive opinion on your rights. My comments above are not intended as legal advice and should not be relied upon as such. However, Apple has a website where you can inquire about legal issues pertaining to the use of their products. That would be free. You may be able to get your answer there. Here's the link.
Legal - Contact Us - Apple
-- Rich