I had almost the same problem. Mine was the volume tuner/rocker after the new update, where Asus implements the Asus Volume Adjustment.
Once i deselected this feature and used back the classic volume control i had no more issue, clearly a bug. (Yes i restarted, reset and rebooted the device nothing changed).Using the new Asus volume feature i got low sound even if it was at 100 % and I don't use smart ringtone .
So yep try this metod , switch to classic leveler in sound options if you are using the new Asus one and see if it works.
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