Yeah, if I enable the Realtek HD audio, sound will only play out of the monitor that is connected via DVI. If I enable NVIDIA HD Audio, sound will play out of the HDMI monitor. I can not enable both at the same time to my knowledge.
I have two ASUS VE278Q monitors connected to my computer, one via DVI and one via HDMI. Both of these monitors have internal speakers and I have used a Y-cable to connect them both to the 3.5mm audio on the back of the computer. However, I am only able to get sound out of one of them at a time...