Auto Audio Switching Not Working?

[Stee93]

I'm trying to test an ASRock X79 motherboard, however the audio is behaving slightly oddly with a fresh install of Windows 8.1: with most motherboards I've used, if no speakers are plugged into the rear port, audio will play via HDMI from the graphics card, but as soon as I plug something into the rear audio port, it will automatically switch to that in preference. But with this motherboard, I have to manually set the rear port as the default device to get it to output there - and then when I unplug, it doesn't automatically switch back to monitor via HDMI either, but again has to be manually selected. I have the latest audio drivers from the ASRock website installed.

 

To summarise, the ports work when selected but aren't causing the usual automatic selection behaviour. I'm concerned that there might be something wrong with the board that's causing this problem. Does anyone know what the culprit might be?