Adding audio is pretty simple once you get the proper add-on. Unfortunately, Slides does not allow you to insert audio files, but with the Screencastify add-on, you can record a video, then shrink that video on the slide or hide it behind a text box, leaving the audio narration playing.
Here is a handout I shared with my students. It includes some simple how-to videos from another teacher who used Screencastify with her class. I'm happy to help if this is something you would like to use in your classes!