Graham Fowler

Inability to access Australia's  ABC Iview is likely due to the need to log into iview with a valid email and password which became necessary a few weeks ago.

https://iviewsupport.abc.net.au/hc/en-us/articles/360003865776-How-do-I-sign-up-for-an-ABC-Account-

On your TV you should get a message saying go here: http://www.abc.net.au/linktv

and a 4 character code to link a signed in device to the TV.

Worked fine for me.

chook

Some info. 

OLED55P TV   Marantz SR5500 receiver.

A call to support revealed sound out using  "Internal Speaker and Optical" option  will  only be stereo. 

Sound out set as  "Optical" only,  passes 5.1 Dolby,  Atmos and DTS  through to  the AMP without fault.  You can use the Dolby apps in the apps library to test it out.  Very impressive.  

I suspect Digital Out will do the same. I don't have a cable to test.  It needs to meet a digital standard.

Try selecting just your SONOS  HDMI output as  ARC as your sound out and dont split it with the internal speaker.  I cant test this as I don't have HDMI input on my Amp but given how the optical  output works it may work the same for ARC. 

Good Luck.