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.