I'm certainly not any more clear about it but do have a couple of observations.
At the bottom of Realtek download page is an ATI HDMI Audio Device. This tells me that in some cases HDMI audio is handled by the Graphics card. ?? Realtek
The other option would be to check any hidden output devices.
Right click on the Microsoft speaker icon, select playback devices.
Next, right click almost anywhere in the middle of the window that opens and select both "show disabled devices" and "show disconnected devices".
What devices are showing and which device is set as the Default Output?