Once you allow the permissions, make sure you right click on the slide and select manage audio. You will then see the options you see below in which you can record or upload (must be an MP3 in Google Drive).
When adding audio you have the option to play the audio on selected slides, or throughout the whole presentation.
Embedded below is a GIF followed by a 2 minute YouTube tutorial. Big thank you to Layla Lyons for sharing this with me!