When you use videos in a presentation, or in general, be sure it has synchronized captions. If you include a video that already has captions, then you do not have to worry about this step.
Section 9.1: Adding Captions to Videos
In PowerPoint, in the Normal view, open the slide that has the video that you want to add captions to.
Select the video on the slide.
On the Playback tab, select Insert Captions, and then select Insert Captions.
In the Insert Captions dialog box, select the file or files and then click Insert.
If you need to add more caption files, just repeat the process.
Play the video and check that the captions appear correctly.
Any multimedia additions to a presentation has to have a text transcript and Alt Text.
Section 9.2: Automatic Captions and Subtitles
On the Slide Show ribbon tab, select Subtitle Settings.
Use Spoken Language to see the voice languages that PowerPoint can recognize, and select the one you want. This is the language that you will be speaking while presenting.
Use Subtitle Language to see which language PowerPoint can display on-screen as captions or subtitles. This is the language of the text that will be shown to your audience. By default, this will be the same language as your Spoken Language. You can change it to a different language, meaning that translation will occur.
In the Subtitle Settings menu, set the desired position of the captions or subtitles. They can appear over the top or bottom margin of the slide (overlaid). They can also appear above the top or below the bottom of the slide (docked). The default setting is Below Slide.
More appearance settings are available by clicking Subtitle Settings > More Settings (Windows).
You can change the color, size, transparency, and font style of the subtitles. You can change the background to improve contrast and make text easier to read.