Thanks, I have managed to play Atmos sound now. Youtube does not support it as you said. But I had to set ATMOS=ON from TV speaker setting first then set output to HDMI Arc (eARC = ON). Without ATMOS=ON on TV the output sound from ARC was just PCM.
I still have the delay when changing volume from TV remote. Pressing the + (or -) several times you can seen the soundbar volume changing slowly. Tried V2.0 HDMI cable but still the same.
My soundbar is SL10Y (not SL10YG). Could this make any difference?