Installing windows 7 on my Zotac H55-ITX WIFI miniboard, I noticed that there is no sound through HDMI port.
First make sure you have the latest driver for you intel graphics from Zotac downloads then restart the computer and go to BIOS settings (hit DELETE when computer boots up).
Under "the southbridge settings" make sure "internal HDMI" is "Enabled".
Login to windows, right click on computer and select Mange. Under the computer management list select "Device Manager"
Now under "Sound, video and game controllers" you should have a "High definition audio...." device added.
Right click on it and select update driver and choose to browse your computer for the driver.
From the Graphics driver you just downloaded select the directory "ibxHDMI" which contains "IntcDAud.inf" file and click ok.
This will install the correct driver fro you "Intel Display Audio Device". (make sure in "Playback devices" you have your HDMI device selected as default.