Voiceover is not being captured in imovie

The next thing to try would we deleting preferences. Open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects.  Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.

-- Rich

Go to System Preferences/Security & Privacy/Privacy and click on the Microphone item in the left hand sidebar of the Security & Privacy box. Then scroll to the iMovie item in the right hand side pane and put a check mark next to iMovie. To make changes you will first need to click on the little lock icon at the bottom left of the box, and then sign in.

Did deleting preferences solve it?

If not, try creating a new project and see if the voiceover works there.

Failing that, try redownloading iMovie by dragging your iMovie app into the trash (but don't empty the trash) and then redownloading it from your purchases list in the online app store.

-- Rich