Bluetooth Speakers for Apple TV (4th Generation) Volume Control
I think Bluetooth will do. The volume can be controlled using the Siri remote. Also, it won't have the problem of sound lagging with AirPlay. 
The only problem is that the Apple TV doesn't have a disconnect option for the Bluetooth speakers (this is a problem if you are sharing the same speakers among your computer and your Apple TV). Once you have connected with your Apple TV, there are two ways to disconnect them:


connect the Apple TV with another Bluetooth speakers so that the first pair will disconnect. 
forget the speakers (but need to pair again next time)