There is no built-in option to add audio clips to a slide in Google Slides but there is a pretty easy workaround.  Using a screen recorder, like Quicktime on a Mac or Screencastify in the Chrome browser, you can easily record yourself speaking over a slide and add that clip to the slide to add narrative audio to your presentation as you share this.  This is something we have done with students as they are sharing their slide presentations and want to add their own audio to each slide.  Follow the directions in the embedded Google Doc below to see how to do this.