Hi,
I am having a similar problem, except I get no sound in any of the addon aircraft (FSlabs, PMDG 737NGXu, 737, 777, MaddogX, ...). I tried reinstalling GSX from zero, reinstalled the sound drivers and DirectX, I also disabled other sound devices in the control panel. Please note I only get only one sound device option in the GSX in-sim menu, which I actually use as primary device. I am not totally sure, but it seems like the installation of the 737NGXu caused the problems in my case, as it was working fine before.
Any advice, further instructions?
Kindest regards,
Matej