So, there's no actual way to insert music into your slides unless you use imovie. BUT, you can place a link in one of your slides to a particular song on spotify, and as you are presenting, you can click on the link, and it will open up spotify and play the song you selected.
Okay so first, go ahead and open up your google slides presentation.
Then find the specific slides you want to add music to.
Select Insert from the menu, and then click on text box. Place the text box anywhere in your slide, and you can put some text in there to disguise the link, or you can just paste the link directly.
TO GET THE LINK:
Go to spotify.com
Pick out the song you want to use.
Right click, and select "copy song link"
Then paste that link in your text box you created!