I tried Duckweed, Gibba and Lotus, none of them gave me HDMI audio. I sticked to Gibba, because it has 5 Ports, as my GC does (DL DVI, DVI, HDMI, 2x DP). However, seems not to change anything.
And thanks, I deleted the second one.
Edit: After deleting the second device and restarting, it...