Voiceover not recording in iMovie

Hi guys. I am trying to record voiceover in iMovie. When I press the red button to record, it start recording "the silence". Input source is well fulfilled (I have tried with built-in micro and system settings) - same problem . In systems preferences, I use Internal Microphone, Input volume is at 100% and react correctly. Mute isn't checked. A record is created but sound still flat.

I did a reset of preferences as suggested in a previous case but problem still exist.

How to solve it?

The app and OS are up to date. My iMac is running in 10.15.2 and Imovie in 10.1.14.

Thanks in advance .

Regards.

Phil.