Apple TV iPhone remote
Need help with volume on iPhone Apple TV remote
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The software remote on iPhone isn’t able to use an IR signal directly to the TV or audio system, unlike the physical remote.
For certain AV receivers (and by extension possibly certain soundbars) however, Wi-Fi devices may send volume control signals to Apple TV, which may be passed on through CEC. If your AV receiver support volume control though CEC from another device (Apple TV box), then the software remote on iPhone can send such a signal using the volume buttons on the side while the remote widget is active.
Else you will need to use the audio system remote or TV remote for volume control.