You are not the only one who has asked about use of iMovie's audio media. Responses have been unclear or at least varied.
Open iMovie and go to iMovie/About iMovie/License agreement. Read the entire license agreement, but particularly the last paragraph of Paragraph 2G that says:
"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)." (Emphasis added.)
The bold faced words that have been emphasized above are newly added. They weren't there last year. There may be a question as to whether uploading to a social media site, thus broadcasting to the public, goes beyond "personal use". I don't know. Also, "royalty-free" does not necessarily mean copyright free.
Copyright laws are a briar patch. Takes a specialty lawyer to figure them out. If you can't persuade FaceBook to post the video your only option is to remove the audio or change the audio to something that is not copyrighted.
-- Rich
