My configuration is: LG: OLED55C1PUB tv (optical) -> SNC4R soundbar with sub and rear speakers.
The LG soundbar only supports Optical for connection to the TV.
When LG TV is chosen as input, (HDMI arc/earc where the Roku ULTRA is not the selected HDMI input) and the Pandora app (for instance) is opened audio is heard from all of the components.
When the ROKU ULTRA is chosen and the Pandora app is chosen, the audio does not come out of the rear speakers. Everything else is perfect. The sub 'subs' soundbar works and the ROKU remote controls the volume. ONLY the left/right rear speakers are muted.
This behavior is consistent regardless of the chosen app.
I have tried several of the HDMI inputs for the Roku ULTRA and all exhibit the same behavior.