You must select the correct audio device first, then restart select "restart Couatl" and be sure it reports no errors.
If the correct audio device is listed in GSX and it reports no errors, then the problem doesn't have anything to do with GSX, it's possible that Windows itself has muted or lowered the volume on its own.
In some cases, Windows itself changes the volume of the application because there's focus on other events like notifications, and this might mislead users GSX has a problem with audio, because Windows has turned it down the volume due to a notification of another app.