Help No volume control over hdmi mx3-g


First of all hi all , and I m new to the forum so if I misplaced the thread sorry in advance.

I have a mx3-g tv box , it came with android 5 connected to the tv via hdmi.
Now the problem is the following when I watch ( from a pen , dlna or hd) if it is a divx mpeg or mp3 I have no problem whatsoever.
When I play an mkv ( whith digital audio tds or dolby) the volume can not be changed .

If I press on the remote the volume down or up button I get the scrolling bar of the volume and it moves up and down accordingly with the button i press but the volume level does not change at all.

I temporarly downgraded to android 4.4 and it all works well without a single issue , so I assume it is and adroid 5 bug?

I also tried android 5.1 same identical issue.

The only settings I get for the volume is ....system sounds on /off , digital audio auto, pcm ,hdmi ,raw which I obviously set to hdmi.

Has anyone encountered the same issue ?

Is there a solution to it ?

Thanks in advance