Hi,
Your rights are determined by the copyright laws and the iMovie license agreement. Paragraph 2G of the license agreement provides as follows:
"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)."
So, the language would suggest that you can use the audio content for your "own personal, non-commercial use".
Publishing to social media may take it out of "personal use". Monetizing it sounds like it might be out of "non-commercial use".
Basically, the only way that you can know your rights for sure is to consult legal counsel. Another alternative would be to consult Apple's legal website and pose your question. Here's the link:
https://www.apple.com/legal/contact/
-- Rich