It may not be the case for everyone, but I also noticed a drop in GSX volume after the update. I managed to fix it.
There were 2 things going on. In previous versions, the slider to adjust the GSX sound volume was not working. I tried it a few times, but it made no difference.
Problem #1 - When it started working, I had a lower volume because I had left the GSX sound volume slider set at a low level when I tried it.
Problem #2 - I had "fixed" the original GSX volume not working problem by adjusting the volume for couatl in the Windows sound mixer.
The solution for me was to simply max out the couatl volume in the sound mixer, and then raise it to a higher volume in the GSX settings.
The GSX sound volume has been just right for me for several flights in a now.