Control Panel Sound and select the default among devices listed. Alternatively if you are not going to use the analog out or front panel ports or mics, disable audio in mobo BIOS and there will be no confusion for Windows.
I use two Ati Radeon 5770 graphic cards MSI and XFX(crosfire, I use the MSI HDMI port, when I use the XFX there is no signal on the monitor. Sometimes the Ati is the default sound device and sometimes the Realtek HD is the default sound device. In fact I do not use Realtek HD, I use ATI with HDMI.When the Realtek HD was the default sound device I checked the sound devices and I found that the Ati Sound device is not connected but it is always connected. To solve the problem is to unplug the HDMI cable and plug it again or restart the computer. So how can I use only the Ati as the default sound device without uninstall or disable the Realtek HD built in on Asus Rampage III extreme.