The Music Playback Full setting has no effect on a TV source. It's only for streamed audio sources, not video. Also, regardless if they are listed as 5.1, most YouTube videos for free accounts are 2.0 stereo.
With higher quality stereo sources, the Arc will perform an upmix of the stereo audio and play what is determined to be ambient audio out of the surround speakers. But with lower quality stereo sources, little to no audio will play out of the surrounds. Unfortunately, A LOT of YouTube videos have low quality stereo.
Although most YouTube videos are only in stereo, there are some available in 5.1 now. So you should hear audio from the surround speakers when playing these videos.
What model TV do you have?
The TV ints KOGAN it's Australia brand
so if I have YouTube premium if it will be heard in the rear speakers???
A premium subscription won’t matter. It is more about the audio quality of the specific video you are watching. It could also be a limitation on the Chromecast too.
Play the video below on YouTube from the Chromecast and let us know what audio format the Sonos app displays, and if you hear any audio from the surround speakers: