[madjockxbmc]

I haven't tested it, but I've heard you need to extend the desktop when using the DisplayPort to handle HD audio.

 Yes that is correct, but due to the way HDMI works, Windows sees it as a 2nd display, this is what creates all the issues, if you disable the 2nd "fake" display you lose audio, if you keep the display, it is a total pain with things wanting to minimise or go to that screen. The only workable fix is to keep the desktop at 2160p 23hz, as for some reason it does want to jump to the other display.