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Is the iMovie track or clip that you are adding a voiceover to, already possessed of audio?
If not, that might introduce a problem.
But first, check on *all* Volume controls.
- for your computer's audio
- for your computer's microphone
- for your software (iMovie)
- for your software's microphone control (if iMovie has one).
In every case, make sure no control is set to MUTE. That will shut off audio.
Note: the symbol for MUTE is the speaker icon with a red-slash over it (and perhaps a red circle around it), and/or an X over it. If you see that symbol or that sort of symbol anywhere, note it, and try to disable it or change it. If it won't change, note where it is very carefully and in some detail and get back to me, please.