You can set up your Apple TV’s Siri Remote to control the volume levels of a separate sound system. It might not work out of the box. But with a little poking around in Settings, you can set it up so you can use the convenient volume buttons built into the Siri Remote to control sound bars, etc.
That means you won’t need to keep two or three remotes lying around — you can make it all work from just one. It just takes a little setup.