Google has added Closed Captioning to Google Slides as an accessibility feature.

When enabled, your computer's microphone will transcribe your speech in real time as captions on the slides you're presenting.

The closed captioning feature is helpful for those that are hard of hearing and can also be useful for quiet presenters or for presentations to a large group.

You can activate the feature by clicking on the "CC" button in the navigation box when you're presenting. There is also a keyboard shortcut to activate it. Use  Ctrl+Shift+C in Chrome OS/Windows or command+shift+c on a Mac.