I had the same problem (no sound on hdmi out), for me it came from the drivers for my graphic chipset (Geforce 9300M). It appeared that the drivers were set to DVI and because they couldn't identify the hdmi cable that i plugged in i couldn't do anything about it. So i started looking for better, older drivers (Nvidia's 182.5 for Vista did it for me, with W7 RTM x86).
I'd recommend you do the same for your Intel graphic chipset.