Same kind of trouble here. I have just built a Intel i5 4570S (HD4600, latest driver) on a Gigabyte GA-H87N-WIFI (rev 1), and I can't get sound through HDMI. In the windows sound settings, the HDMI output says "not connected", and there seems nothing I can do about it, even when playing with device manager. I have installed and uninstalled Realtek drivers with no improvement. Everything works fine through a jack to PC speakers. Please fix this rapidly as an HTPC with no sound output is rather useless :-(.
thank you in advance for your help