You can set the speed of Standard voices on the Voiceover tab of the Storyboard. (Premium voiceover speed is not adjustable).

From the storyboard, select Audio and then navigate to the Voiceover tab.

Select Speed to adjust the speed of your voiceover.
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The speed is set to normal by default. Drag the slider left to decrease or right increase the speed of your voiceover.

The new speed will be displayed as a percentage next to Speed and is automatically applied to your video.
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To view your video's updated duration, select Preview.