Whew! That was a bit of a complex problem to solve!
I recently bought a Roku Premiere+ streaming device to replace the old functionality on my 9 year old Blu-Ray player. The DVDs and Blu-Rays still play back fine, but it's starting to be a hater with the ever-increasing quality of video streams on its aging processor and Netflix and Amazon Prime Video apps. Google Chromecast does not have Amazon Prime Video support. So, alas, it was time to research a streaming device. CNet strongly recommended the Roku, and so I picked up the new Roku Premiere+ with Ethernet connectivity.
I have a B&W surround sound 5.1 (well, 5.0 -- I live in an apartment so no subwoofer for me to keep the peace with all my neighbours!) speaker setup -- a L, a R, a center channel, and a rear R and rear L.
In the last while I've been using my old-ish (maybe 7 yrs?) Pioneer VSX-919AH receiver more and more in the Advanced Audio mode called "External Stereo" which is a sort of simulated surround sound. I find it works well enough to use all my speakers (and better so than Dolby PLII or the other options). I used to use it when playing back audio in stereo (eg Pandora) but wanting it to come out of all the speakers. Fair enough.
Recently, I've noticed that the Netflix and Amazon movies tend to come through only in Stereo. I thought that they just weren't streaming in 5.1 very often. Turns out... my setup was a fail for 5.1 streaming!!
The issue stemmed from the fact that I was connecting most of my devices (eg Chromecast, OTA Digital Antenna, etc) to the TV, and then passing the audio for these channels back to the Receiver using a Digital Optical Audio Cable. Normally, that should be fine.. but...
Apparently in the last few years Netflix, Amazon and other apps on Roku (and presumably on other devices) now only serve Dolby 5.1 Surround Sound through Streaming Dolby Digital PLUS. Dolby Digital (non-Plus) works fine via Optical Audio Cables. But Dolby Digital Plus DOES NOT TRANSFER over an Optical Audio Cable. You MUST transfer it over HDMI. And since Netflix et al only serve 5.1 in Dolby Digital Plus (DD+), if you're not serving your entire audio/video pipeline with HDMI, then say goodbye to glorious 5.1 streamed audio. Crazy.
I had followed what I was doing for the Chromecast and plugged the Roku into the TV HDMI input, and then relied upon the HDMI switching on the receiver to help me along with the Optical Audio Cable from TV->Receiver for the audio. After not finding 5.1 options in any of my Roku apps and getting insanely frustrated, I realized the Roku had auto-detected Dolby Digital (DD) and not the option I thought it would have picked (Dolby Digital + / DTS). I initially thought "Oh no, my receiver is too old and doesn't support DD+! I don't want to have to buy a new one!" A quick Google showed that my receiver certainly supports DD+. More Googling explained that Netflix, Amazon et al now only serve 5.1 audio via DD+. And DD+ does not work over Digital Optical Audio cables.
An insanely simple one, after all that. Sometimes the hardest stuff is just a "one line fix" as we'd say at work :)
I simply removed the HDMI cable from Roku->TV, and plugged it from Roku->Receiver HDMI input 1. Receiver HDMI Out was already plugged into TV HDMI 1 IN, since that's how I send Blu-Ray signals to the TV. So I literally needed to only swap one cable. I couldn't believe it.
Some testing and the Roku now auto-recognized Dolby Digital+/DTS and I was off to the races. A simple update of my Logitech Harmony settings to set the HDMI inputs correctly now, and that's all there was to it. Only took me 4 hrs :)