So far, determined FSX must run in windowed mode for Pilot2ATC to be on top. Realized clicking on window pauses background app (FSX). fixed that with pause on task unchecked. but... pauses the sound output of sim. cant find in windows 11 how to keep background app (FSX) sounds to keep playing. i really would rather run FSX in full screen mode by the way. but doesnt let Pilot2ATC to be on top. what can i do?

https://youtu.be/qngzkLPh7n4?feature=shared 

The FSX Sound options have an option to Mute on Task Switch which you need to uncheck.

There's nothing that will let P2A display over FSX in Full Screen mode.  In that mode, FSX is controlling the monitor and blocking other Windows programs from being displayed.

Dave