I can't enable the sound os HDMI in Maverick with a ECS GT240. Why this not work "out of the box"?
first check if there is any HDMI device listed after using:
if there is anything with "HDMI" select Sound Preferences -> the Hardware and from the list below you must select something like "Digital Stereo HDMI output" and it should work.
If that didn't help and you have nvidia GPU then this should help you : http://forum.xbmc.org/showthread.php?t=69479
Open the Sound dropdown (from the panel), select Sound Preferences..., click the Output tab, and select the radio button next to the HDMI Audio out device you require audio out on.
However if your HDMI device is not listed then the device is not properly installed or acknowledged by the sound server.