If your slide show is longer than one song, you can add more songs. Play multiple songs across multiple slides
Play in Background makes the audio file start automatically during a slide show and also makes the audio play across multiple slides. With the audio icon selected on the slide, on the Playback tab, select Play in Background. In the file explorer, locate the music file you want to use and then select Insert. On the Insert tab, select Audio, and then Audio on My PC.