Apple TV control external HiFi - difference between direct control or Airplay
In order to control the volume of your HiFi, you will need to use HDMI-CEC between your AppleTv and your Receiver/HiFi. Data (audio) transmitted over optical is sent at a fixed volume, and relies on the receiver of the signal to control the volume. 

If your receiver does not have HDMI with ARC, you may be able to get around this in the following way:


Connect AppleTv to TV via HDMI
Connect TV Audio Out to your Receiver.
Configure HDMI-CEC to control the volume on your TV from the AppleTV remote.
Configure TV Audio Out to be "variable", not "fixed" or "line level". This step will vary wildly depending on your TV.


There will be tradeoffs depending on your TV &amp; Receiver's input/output options.