I recently bought a HP Proliant N40L microserver and intended to use it as a HTPC. Once I have discovered that it does not have a integrated sound card and a very poor graphics card, I bought a Gigabyte Radeon HD 5450 Graphic Card and a sound card for it. Accidentally, I found only with the graphic card along will provide audio to TV through HDMI, without installing the sound card. This seems strange to me because both the micro server and the graphic card don't advertise a sound card.
AMD Radeon HD cards have, for a while, supported audio out through HDMI specifically for the purpose of using them in HTPCs. There is a sound processor on the card itself that enables this to be possible.