I have all volumes except air conditioning set to MAX (Both X-Plane and IXEG), and I am running the latest version if the IXEG 733 and X-Plane 11.50.
Sometimes during the landing I can barely hear the GPWS callouts, also they are hard to hear even with the engines off.
My windows volume is set to 60 and X-Plane is set to MAX in the sound mixer.
I would suggest that you add a GPWS volume slider and boost the volume a little bit.