Hi I know this was a few weeks ago so you may have sorted it now.
The reason you aren't getting sound is because your TV/Monitor is expecting to get the sound from the HDMI cable that you have plugged in. Not all graphics cards will provide sound over the DVI or even HDMI ports without first getting the sound from your motherboards sound chip.
I have just purchased two Samsung 27@ TVs to use as monitors and when I connected both via DVI-HDMI cables to my graphics card (I have a GeForce 9600GT) I had no sound (I expected that to be the case, however). I used the 3.5mm jack cable from the monitor to my sound card but there was still no sound. I had to go into the TVs service menu and activate the analog audio input on HDMI port. This...