The problem came back to my machine too...(it disappeared for about a week after I installed the latest realtek driver using updater software called DriverEasy). Now my only workaround is just to turn the volume balance of the right speaker all the way down to zero. After that the sound fluctuation problem disappeared.
My right speaker distorts quite a bit more than my left at high volume, so really start to suspect now that it's a built-in mechanism by ASUS to prevent the defected speaker from being damaged further.