I've just build a new work machine with a Sandy bridg CPU (i7 2600) and a Cougar Point motherboard (Gigabyte GA-H67MA-UD2H-B3). This uses a Realtek HD Audio codec. I'm using dual monitors (but the issue is still present if I only use one) with one DVI -> DVI and the other HDMI -> DVI and taking the sound via a 3.5 mm jack.
The default playback device is showing up correctly as 'Speakers' and when I insert the 3.5 mm jack I get the pop-up telling me I have speakers plugged in. When I test the speakers it shows that a sound is playing but nothing comes out of them.
I've already upgraded the driver to the latest Realtek one and tried using a single monitor over DVI but nothing has worked.
Does anyone have an idea what may be going on?