I've done a search on this topic, but none of the solutions listed work for me. I also noted that there is a thread with a similar title two posts down from mine as I write, but it regards audio through ARC, which is not my issue. I've also seen the thread Roku OS 10.0 Update Breaks Surround Sound, but it's not clear if that is a universal problem on all Roku devices and that thread appears dead with regard to Roku support input.
The following is my situation. I just installed a new home theatre system centered around a Marantz AV7706 pre-amp/processor into which I am using a new Roku 3810CA Streaming Stick Plus as my primary media streamer. Surround sound does not and never has worked on this device as far as I can tell. The Roku reports that firmware is up to date at version 10.0.0 4195-50. Audio settings are Auto for HDMI and Audio Mode is Auto (DD+(DTS HD)). Regardless of these settings, all content comes across as Stereo only. Netfliix and Prime in particular. Is there something else I'm missing in setup?
Just to close out this thread, I followed the advice above and I still could not get surround sound to work. I tried numerous other troubleshooting tips on this forum and elsewhere and nothing worked. So, the only solution that worked for me was to dump the Roku Streaming Stick Plus and replace it with the new Apple TV 4k Gen 2 that was just released. It works flawlessly so far to the point that I don't anticipate ever being a Roku customer again.