So it seems I can't get 5.1 from Chromecast Netflix app because, like Roku, it uses Dolby Digital Plus which the PLAYBAR doesn't support.
To summarize what Netflix devices can pass 5.1 to the PLAYBAR:
Apple TV - yes
Netflix built-in to TV - depends on TV
Chromecast - nope
Not related to Netflix, but I have another strange issue as well. If I plug the optical out from my Blu-Ray player directly into PLAYBAR and play a BD movie with TrueHD soundtrack, the Sonos shows 5.1. If I connect the BD player via HDMI to TV, then TV optical to PB, I only get 2.0 even though the TV can definitely pass 5.1 from external outputs - it works with AppleTV Netflix. So is there some kind of copy protection thing going on here, either at the BD or TV level?
My head is spinning trying to figure out what will and won't work.