Apple TV Remote App on iPhone Volume controls are greyed out
The volume controls switch off when I select the apple TV in the iPhone Apple TV Remote. They used to work, all of the sudden they are greyed out. No settings have been changed.
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The volume buttons on the side of iPhone can send volume control signals through Wi-Fi to the Apple TV box, which will then passthrough that to equipment that will listen to HDMI CEC volume control signals from other devices. In general, that only works for select AV receivers that have support for this. Not all equipment with CEC support will take CEC volume control signals from other devices, notably not TVs. It also requires that Apple TV is set for Volume Control through HDMI. (On my older Pioneer receiver, it requires a click per 0.5 dB step; continuous pressing doesn’t take it further.)
In contrast, the Siri Remote would send IR volume control signals directly to the audio system, be it receiver or TV or other, that can be volume controlled by IR. (iPhone can’t send IR signals.)