The problem with that gets back to Windows only allows one "default" audio playback device. If HDMI port 1, for example, is set as the default audio playback device and you are also using HDMI port 2, you would not be able to set it as a playback device. This is a Windows limitation on only one default audio playback device.
Can you post a screen shot of the Sound panel/Playback section?
For example, here is mine. I'm not using HDMI, just the PC's sound card (Speakers).