There is not currently a way to directly insert audio into a Google Slide presentation. This video shows a work around using the Screencastify extension (https://chrome.google.com/webstore/detail/screencastify-screen-vide/mmeijimgabbpbgpdklnllpncmdofkcpn?utm_source=chrome-ntp-icon).
It is also possible to create the original audio file using Screencastify or YouTube Live with a black background.
In this case, YouTube Live is blocked for student access, and the teacher had already recorded the audio file on her phone.
