Audio Disapearing when playing 7.1 movie on 9.1 surround system.

Hi there,

I got a Samsung TV, series 8. UE55JS8005TXXE
Latest version of Tizen installed with latest version of Plex.
The surround system is a LG 9.1 Channel BH9540
Connection is HDMI throuh the ARC port.

Playback of MKV files with 7.1 channel sound, results in no sound.
A blue ray disc with 7.1 works fine.

Any solutions for this?