Title of imported song in Audio and Video tab of iMovie can't be removed

I imported a song into iMovie from my Apple Music library that had originally been copied from the source CD. I didn't want to use it in a project, so I went into the Audio and Video tab to remove it. There's no way to remove it. I tried doing the right click on the title to reveal in finder. Thinking it was just going to remove a file copy created for iMovie, I trashed it. Still didn't remove the title from showing up in the Audio and Video tab, although the song was no longer accessible in iMovie - and the unintended removal from Apple Music as well. I got it back by going into Trash folder, but the only file I could "put back" had a different file name - not a problem really, but at that point I was entirely confused. And the file title STILL shows up in iMovie's Audio and Video tab - can't I get it out of there? Really? What if I had a thousand songs imported and wanted to remove the reference to them - they're there forever?? Thanks for any advice.