Not all video cards can pass audio through the HDMI cable.
If your monitor/TV has regular DVI/VGA and audio inputs, the simplest solution would be to use those connections instead. (There is no performance advantage to using HDMI over DVI.) Btw, my HTPC connects to my 47" HDTV using just the VGA and audio connections, and it works just fine. (I had it connected via HDMI, but I then stole the cable for use on my Blu-Ray player and see no need to replace it.)
Otherwise, you may need to find out if your video card can pass the audio and/or what drivers or connections may be required. What video card (or on-board video) do you have?
Last edited by Quaker; Jan 12, 2011 at 04:55 PM // 16:55..