The Volume button and mute button no longer work on my att unverse remote, S20-S1A. I have no idea what happened. All other buttons are working just fine.
@Das6105 - Maybe there is something blocking the TV IR sensor. But, that would probably also affect on/off.
Try reassigning the vol/mute - https://www.att.com/esupport/article.html#!/u-verse-tv/KM1050800