HDMI audio doesn't work through DisplayPort adapter on OS X but works on boot camped Win 7
Hold down the option key and click on the volume icon in the menu bar. What does the menu say for Output Device? If your adapter doesn't appear, here's what you can do: Open /Applications/Utilities/Audio MIDI Setup. See if your adapter appears in the list to the left. If it does, play with the Configure Device and Configure Speakers options (accessed by the gear button).