I recently bought the adapter from monoprice that puts USB audio and miniDP on an HDMI cable.  I don't have any problem getting the signal to my TV and receiver, but I can't control the volume at all with the system volume adjustment on my mac mini.  I push the buttons on the keyboard or the tool and nothing happens at all.  The volume stays the same.  I can however control the volume through applications i.e. iTunes, Hulu, VLC.  What gives?  Is this a "feature" built into MacOSX.  BTW, I can control the volume with the system if I run audio out from the mini to the input on the TV.  Any ideas?