Ok, I updated audio driver to the newest version (Realtek HD Audio v2.79) - will see...
As I've said in another thread, I just rebuilt a new PC from scratch, with Windows 10 freshly installed. Using the Windows 10 audio drivers that came with the mainboard ( ASUS, with Realtek audio chipset ), NORMAL WINDOWS AUDIO randomly disappeared. I'm not referring to GSX or FSX. I'm referring to the normal, standard, sounds that Windows makes.
Searching online, it seems Realtek is blaming Microsoft, and Microsoft is blaming Realtek...
However, when I uninstalled the audio drivers that came with the mainboard, and replaced them with the generic ones that comes with Windows for the Realtek chipset, all my problems with audio disappeared.
I'm not suggesting this might be the same thing that affects Couatl, but it's something you might want to try.