DisplayPort oddities

[Voodoo90]

Hello,
I have a kinda strange problem.
I am using Ozmosis and a R9 280X graphics card. So far I have used the DVI and the HDMI Out on the card. Without problems. But I wanted to enable the Sound on my monitor. And since DVI doesn't support sound, I want to connect it via DP. So I bought a DP cable.
Now the strange thing.
When I connect the HDMI and DP, nothing happens, I have a black screen on the monitor, only HDMI works.
When I connect only the DP nothing happens, black screen.
When I connect the DP and the other DP with a DP to HDMI adapter I have both screens working and I can enable the Display Port audio over the native DP cable to my monitor, but then HDMI audio ist not working anymore.
I need both, they are very important. How come the DP cable only works the adapter is connected.
How can I fix this? Buy a new cable? Anything else? On Windows it works just fine.

[Voodoo90]

Meanwhile I swapped my R9 280X for a Gigabyte Rev 2.0 version.

But still, I can use DVI on its own.

HDMI Output isn't working at all.

Display Ports work, but only if one is used for a HDMI adapter. I can't use one Display Port on its own for my main monitor.

 

Is there nobody, who can help with this?
I/O Jones says all 4 connectors a detected.

So where is the solution to this.

Best solution would be main monitor to DP and TV to HDMI, both with Sound.

 

I'd be thankful for every hinch.