No sound RS520 with RoseTube over HDMI

I own a RS520 and connect this over a HDMI to my TV to use RoseTube.
I finally got video working there is just no sound coming out.
In the app when trying to volume it says something like ‘In/Out settings not correct’ but I have selected HDMI as output? Whenever (as a test) I just select an other output the video stops, so I think the output over HDMI is also selected correctly. I just am not able to get any sound?

@Joop

To connect the RS520 to a TV and watch RS520 video on it, you don’t need to change any input/output settings.

After setting the input/output settings, simply press the TV On/Off button in the upper right corner of the video playback screen on the RS520 to turn it ON.
Please check the video below.