The volume buttons respond differently depending on how you have your ATV connected and programmed.
For example if you just have a TV then they could adjust the TV volume. If on the other hand you route the ATV through a audio receiver then the receiver volume could be controlled. If un-programmed they may do nothing to the TV or receiver.
Describe your setup and connections and what speakers you are using.
Include the TV and receiver brand and model numbers to help us identify which features are available to you.
Also have they ever worked and just stopped or have they never worked?