In this case, to add Google Slides with music, open the presentation, choose a slide, and menu button Insert / Audio, and a music file that is on Google Drive (MP3 or WAV) is selected and an audio icon appears on the slide. The playback games in the Format options panel can be set to play automatically, loop or hide the icon at the slideshow. You can also have some background music going on, in which case you can add the same sound to the first slide and have it come on automatically on all slides.