The volume control is no longer working on 3 of my Apple TV

1. Try restarting just your Siri Remote. If the volume buttons on your Apple TV remote aren't working - Apple Support
2. The Siri Remote could send IR volume control signals directly to the TV or other connected audio system, skipping the Apple TV box. Verify that you have a clear line-of-sight from the remote to the IR receptor on the device that handles the audio.
3. If the Volume Control﹥Auto setting isn’t working for you (it should work for most users), then consider changing it manually to TV via IR, or Receiver via IR, or Soundbar via IR, whichever applies to your setup. Configure the Apple TV remote to control the volume of your TV, sound bar, or receiver - Apple Support
4. Verify that your Audio Output is set to ‘TV Speakers’ (= over HDMI). Specifying a separate sound system here would use stereo AirPlay or Bluetooth, which is probably of lesser quality than when the audio signal goes over HDMI.
5. Volume Control over HDMI (using CEC) is only supported by select AV receivers. To date, no TV model has been identified that supports receiving volume control signals from other devices via CEC. Not sure if and which soundbars support it.

if it was me then I would disable hdmi-cec on both appletv and the Samsung and then I would pull the plug on both and let then start up again and then reenable hdmi-cec