Storyline 360 – How Do You Add Subtle Background Music Without It Overpowering Narration?

Hey everyone 
I’m trying to add a soft background track under my narration in Storyline 360, but it keeps sounding too loud or cuts off weirdly between slides.
Is there a way to make it play smoothly across slides or fade in/out gently?
Looking for simple ways to balance music and voiceover.