You can set the speed of Standard voices on the Voiceover tab of the Storyboard. (Premium voiceover speed is not adjustable).
Here's a step-by-step guide on how to adjust the speed of the Standard AI voices.
On the storyboard, select Audio and then go to the Voiceover tab.
Scroll down to the Standard voices and click the speed button.
Be sure to apply a voiceover first as you can't adjust the speed of a voiceover without applying one.
3. The speed is set to normal by default. Drag the slider left to decrease or right to increase the speed of your voiceover.
4. The new speed will be displayed as a percentage next to Speed and is automatically applied to your video.
5. To view your video's updated duration, select Preview.
And that's how you update the speed of your voiceover.