Is it legal to post a video with the soundtrack that has been on iMovie???

iMovie legal from June 2021

http://images.apple.com/legal/sla/docs/iMovie.pdf

2.G "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)."

I will let you and your lawyer decide if what you want to do fits within those guidelines.