Storyline doesn’t allow you to add background audio or span audio across multiple slides. If you'd like to suggest these features, submit a feature request to our product development team. We take these requests seriously. In the meantime, you might want to use Articulate Presenter if you need to add playlists to your content.
If you're importing a PowerPoint presentation into Storyline that includes Articulate Presenter playlists, the audio playlists will not be imported. (Slide audio will still be imported.)