I recently put together an HTPC with an ASRock B75M-ITX motherboard and a i3-3225 with HD 4000 graphics. On first boot, the display adapter shows uses the Windows standard VGA driver, and the audio adapter simply states "High Definition Audio Device." At this point, I know that I can get audio over HDMI because I can hear the Windows startup sound, and I also played a few videos on Youtube which played through my TV just fine. When I looked at my playback devices, there was an output device for the TV in addition to the speakers and S/PDIF outputs.
After installing the video drivers that came with the mobo (2618), the video/audio devices updated to "Intel HD Graphics 4000" and "Intel Display Audio", respectively. After installation, I couldn't get audio over HDMI anymore and my playback devices only showed my speakers and S/PDIF outputs as options.
Then I downloaded the latest drivers (2761) from the Intel website, installed them, but still no audio over HDMI. If I roll the drivers back all the way, the TV output device appears and I get sound again.
If both the driver provided by your computer manufacturer and the generic driver on our site are having the same behavior, the problem could be related to the operating system you have installed. What is the operating system being used?