I think I found a fix!
-Play a scene from your iphone where there’s music.
-Press the side on/off button on your iphone
-Press the side on/off button on your iphone again and unlock your phone.
-(music should be playing as normal)
-To make sure it continues to play with future scenes/music changes, YOU MUST THEN EXIT YOUR STORY (top left transparent door thingy)
-Preview your story again and everything should be working fine.
Hope this helps!