Updated 4/10/19 attempting to fix loading issues. The current variation only shows if you are in Holy spec as a priest while in combat. Otherwise, it will not show. I suggest also toggling it to show only while in encounters, to avoid it going off during the Tortollan WQs :) However, this option will only activate it when you are in a boss encounter be it raid or dungeon.
Updated again 2/8/18! Now shows grey icon when on cooldown with a timer. Also will re-ping if you have not cast PoM in a timely manner, but will not give you an audible alert during Hymn. (Thanks Redfox)