HDMI/Displayport audio control for OS X
You can't use your Mac's system volume control to manipulate the volume on an HDMI/DisplayPort interface.

Basically, the HDMI signal includes digital audio that was encoded in PCM (Pulse Code Modulation) format.  There are two main components to PCM and those are bit depth (how much of the audio signal is sampled) and the sampling rate (how many times per second the data is sampled).  There's nothing in there that handles volume.  

To put it another way, does your HDMI source like an XBox, Playstation or Blu-Ray DVD player have a volume button or does your TV handle the volume for you?

In order for you to control the volume from your Mac, you would need to do one of two things:


Obtain or develop software specific to your television/display that would embed control signals onto the HDMI signal (similar to Sony's Bravia Theatre Sync
Use a [USB Infrared Transmitter][3] to send volume controls from your Mac to the TV/Display.  Note:  The only software I was able to find that works with USB IR transmitters is IR Control


However, based on the price alone, I would probably just stick to my universal remote and be done with it.