How to turn down and turn up volume
How to turn down and turn up volume when I use the Apple TV Remote app
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As the software remote on iPhone or iPad doesn’t have IR capabilities, the remaining method is through CEC. Volume control through CEC is supported by select AV receivers and select soundbars, but not by TV sets, afaik.
If supported on your setup, then one would use the volume control buttons on the side of the iPhone or iPad while the software remote is active.