Voiceover volume slider is locked at 0 when using USB interface

It is possible that your mic may not be compatible with a Mac computer, since it won’t work on either of your Macs.

Try calling Apple Support and see if the savvy techs there can figure it out. They have the capability of remotely accessing your computer. Click on the Contact Support item at the upper right of this forum’s screen. Navigate to a place where you can enter your phone number. If ou have time, please post back with your results.

— Rich

As an added note, I tried my iMovie voiceover with an external mic (Logitech) and the volume slider worked fine. I didn't need to select it in the AudioMIDI settings. I was not able to replicate your findings of a slider locked at 0. I only mention this to indicate that the voiceover can work with an external mic. It is puzzling as to why it won't work with your particular external mic.

Anyway, good luck with this. Hopefully the Apple Support people will be able to figure this out.

-- Rich

Go to Applications/Utilities/AudioMIDi Setup. Is your external mic selected there? If not, select it. See if that solves it.

-- Rich