The sound and music libraries in iMovie, GarageBand, Logic, etc. are all available to use in your own work. It gets confusing sometimes because yes, the libraries themselves have copyrights attached to them, but the licensing agreement says you can use them as a part of your own productions. What is prohibited is reselling and distributing the standalone sounds and music. (ie. reselling as your own library, taking one of the intro songs and releasing as a single). To quote Apple.....
"iMovie contains sample content including but not limited to graphics, audio clips, video clips and/or
templates (“Sample Content”). This Sample 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 Sample Content included in iMovie may be used on a royalty-free
basis in your own video projects but may not be distributed on a standalone basis. "
Additionally, if for some reason there was a Content ID claim against you by someone who used the same audio clip from iMovie, you would still have the right to use it because they cannot claim these sounds as their own.
hope this makes sense!
Thankyou soooo much. Glad to here it, too