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.
However, if you would like to edit it further, this is how you do it.
1. Click on Adjust voiceover or Adjust as in the screenshot below
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