remote volume control doesn't work when tv is off

Try this: turn the TV on with the remote but block the signal from the remote, so that the TV stays off and the Playbar is still playing a music source. Can the remote control operate the music volume now?

Played music. Closed my hand around Samsung one remote.. My tv switched on.. music still playing.. pressed power button again. Tv switched off. 

(Oddly enough: It is working but seems to stop .. seems like it works for a few seconds after TV turns off)

That’s not quite what I suggested. 
 

Do you have TV Autoplay turned on or off?

Hi. Yes. I usually have it on. (Maybe i misunderstood your suggestion. How do i block the signal of remote?)

Well, the tv won’t turn on if you successfully block the signal, will it? Put something over the emitter to block/absorb the signal, or go somewhere out of range so that the tv doesn’t receive the “turn on” command. Then, the Playbar will still be playing music and you can go back into range and see if the volume control works. 

If the remote is ir. I suspect it is bluetooth, though.