For any potential buyers out there, the Nintendo Switch only supports 5.1 LPCM, which can't be decoded by the Playbar and Playbase. It is likely getting downmixed to stereo and then getting sent to the Sonos. Also, the Switch will likely not add any Dolby support, as the Wii U before it could also only output 5.1 in LPCM. Sound will still play out of the rear speakers and the SUB, since Sonos matrices those to create "fake 5.1".
The one potential upside is that the Nintendo Switch menu has an option to select "Surround Sound" as output. There is a very slight possibility that it might try to downmix the 5.1 source into 2.1 on the fly, mixing the surround information into the FL and FR channels but that's highly unlikely. That comes with the potential theoretical downside that if you select Surround Sound as output, you're losing some of the surround channel data since in a 5.1 encoded audio, a lot of the signal is encoded into the front center speaker with a lower level going to the front-left and front-right speakers.
So what do people have the TV Sound setting on, Surround Sound or Stereo?