I guess, SU12 breaks something here, cause without the SU12 update, everyting was fine on my system :-/
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.