If you are using an uploaded voiceover to auto-sync with the video, then the AI does a really good job with the auto-sync.
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However, if you would like to edit it further, this is how you do it.
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1. Click on Adjust voiceover or Adjust as in the screenshot below
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2. To adjust the voiceover, select the scene > use the green lines to adjust the voice by moving it to the left and right > Click on Save then click on Close