I have my Samsung UN65HU9000 display running at 3840x2160 @60Hz. When using it at this resolution, audio sometimes works, sometimes doesn’t work. If I switch to 1080 or lower resolutions, sound works flawlessly. This happens in Windows & Linux.
I’m using 2x ASUS GTX 980 with a Samsung UN65HU9000 plugged directly to the card with a high-speed HDMI cable.
Edit: Turns out its an issue with HDMI 2.0 Samsung TV’s.