Minibox as audio level control

I have a new SUPERSONIC TV that cannot respond to Cox MiniBox remote control, so I would like the minibox to control the audio level. When I press the MiniBox Remote  MENU button and drill down to the Audio Variable selection, nothing happens after I press SELECT to select the VARIABLE option. I have three MiniBoxes with three different TV's and none of them will do this. Is this a non-viable function or am I doing something wrong?

Thanx.