Doing a voice over in Google Slides is easier than what you expect. First, you have to navigate to Google Slides.
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Then, you should click where it says "Insert".
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And the, click where it says "Audio".
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Now you'll see you Google Drive folders. Find the one your audio is in, and once you find it, click on it.
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Then, you should click on "Insert".
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The voice over audio will be added to your presentation. Now, you can change the configurations however you find most convenient.
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You can decide if you want it to be automatically played, the amount of volume you want, and if the audio stops whenever you change the slide. And that's all!. Pretty easy, right?