i have a new Intel Mainboard DH67GD with the latest BIOS version (BIOS-Version/ Intel Corp. BLH6710H.86A.0110.2011.0415.1506, 15.04.2011).
I have also the latest grafic(18.104.22.1681) and sound (22.214.171.12463) driver for Windows 7 x64. HDMI with sound works fine. I have bought 2 different displayport to hdmi adapters, but with both i have no sound over displayport (video is ok). I tried different BIOS settings (primary video port diplayport ON and OFF, audio over hdmi/displayport = ON and OFF) with no luck. The Realtek HD Audio Manager shows only digital Output and digital Output (optical).
The Windowws device manager shows a Intel Display-Audio device.
i guess, display port can give only video, not audio. In HDMI, you can get both audio and video. Also intel DH67GD motherboard has HDMI port and you can use it. You need not use a Display port to HDMI converter for this.
Since my experience is limited to using a VGA to DVI-a adapter I hesitated to respond. I feel it would be safer to use a graphics card with a display port than deal with the differing logic of monitors.