So, not sure if this is phone specific or not (LG V30), but as I was sitting at my desk at work today, I began hearing a baby crying from my pocket. (Not Normal)
To my surprise, I unlocked my phone and stared at my normal home screen. It seems that unless you CLOSE the app by clearing it, if sound is enabled it will continue even if you open something else or turn the screen off.
On another note, the sound had been cutting out and turning itself off while viewing the app, so this is a step in the right direction there!
App Version 1.2.32