How to connect Apple TV 4k to a sound bar and control volume from Apple TV instead of TV
How to connect Apple TV 4k to a sound bar and control volume from Apple TV Remote instead of soundbar Remote
iPhone 6s Plus, iOS 10
Question marked as
The Apple TV Remote should work for volume control for many devices out-of-the-box. However, some devices may not work with its pre-programmed settings. If the soundbar has an IR receptor for a remote, then the Apple TV Remote can learn that device for volume control.
Control your TV or receiver with your Siri Remote or Apple TV Remote - Apple Support
Settings﹥Remotes and Devices﹥Volume Control﹥Learn New Device.