Error sound pressing record voiceover in iMovie

I'm trying to record a voiceover in iMovie but the record button is greyed out. If I click it I just get an error sound. I have already tried the following:

1. Clicking the options sliders and manually selecting "built-in microphone".
2. Verifying in my computer's audio preferences that input is set to "built-in microphone".
3. Restarting my computer.
4. Holding option and command while re-opening iMovie to delete preferences.
5. Updating my operating system.

Those are all the things I could think of or have seen on forums. Any other ideas? I know my built-in microphone is working fine because I used it in Logic Pro just minutes before I first opened iMovie. Logic Pro has been closed during all of the above.