Volume reducing automatically in apple tv 4

Try the following steps for the Apple TV 4, check whether things are working after each step where appropriate, before trying the next.

1. Restart the Apple TV (Settings > System > Restart).
2. Restart the Apple TV by removing ALL the cables for 30 seconds.
3. Restart your router. (Also try removing it’s power cord for at least 30 seconds)
4. Reset the Apple TV (Settings > System > Reset > Reset all settings)
5. Restore the Apple TV (Settings > System > Reset > Restore)

Try a simple reset, either from the Settings menu, or by disconnecting the power cord for about 30 seconds.

Is the remote charged?

Can you try disabling the Siri volume control? You can turn it off right in the settings where you trained it.

And please describe your audio configuration. How does the audio get from ATV to the air? There may be some other device in play.

There have been a couple of other threads on similar problems, but aside from a rogue remote, I cannot recall the solutions.

How have you configured the Siri remote volume control?

Have you enabled Reduce loud sounds in Settings>Audio and Video?