To add the audio files, you must first make sure that you have the files stored in your Drive. I suggest creating a folder with a name associated to Sound or Music. There are hundreds of free MP3's and WAV sites on the Internet.
Once you have the files stored in your Drive
- Click Insert/Audio
- Select the audio file in your Drive
- Hover over the icon to see playback controls
- Click Format Options to see options to change such as playback options, volume, looping, hiding the audio icon, or replacing the image with something different
This is an awesome change to Slides and music to your ears.