In the Audio settings, under Voiceover:
A Mac voice
- Type a script in the box.
- Choose a voice. Higher-quality voices are marked enhanced.
- Click Add Voiceover. Change the script and click Speak Again to redo it.
The voiceover starts when the video starts. It's made on your Mac; nothing leaves it.
Your own voice
Click Record My Own Voice. The video plays without sound so your microphone only hears you, and recording stops when the video ends, or when you click Stop Recording.
Voiceover volume sets its level, and Remove deletes it. A new voiceover replaces the old one.