I had the same problem. I was running the PC into my HDTV via dvi to hdmi cable and the sound into my TV via stereo cable into the input for audio when using DVI. (One of the HDMI inputs on my tv has a stereo input for users running a device via DVI.)
Either the PC or my TV handled things incorrectly. Anyone have an idea as to why?
If you're doing something similar you may need to use speakers designed for a PC. I'd try it thru my stereo receiver but it has a channel out and I don't feel like hooking it all up just to not be able to use it =P
Hope this helped.