Sonos Sues Lenbrook for Bluesound and BluOS, Alleging Patent Infringement

Echo also does Sub bonding to the stereo pair, but all of this bonding only works for streamed music; it does not work when music is sent via bluetooth from a device. And of course, there is no way to get music from a NAS.
It would appear therefore that this bonding is done at the source, in the cloud.
I reckon that the multi room groups are also handled in a similar manner.

And of course, there is no way to get music from a NAS.

Hey Kumar, small point (and apologies if you already know this) but you can stream local music to an echo with Plex.

I don't think you can stereo pair echo dots, but you can pair echos together, at least on the 2nd and 3rd generation units, along with a sub. Because you can do multi room with all the echo versions, but pairing only on some, leads me to believe that the pairing method is different than multi room in this case. As well non-Amazon alexa devices can be in the alexa multi room group (if they aren't part of their own multi room system like Sonos) but can't be stereo paired.

Also from what I've read, although various echos can play audio from non streaming sources through bluetooth and other means (as @Kumar stated), only streaming sources can broadcast for multi room. I don''t know if that means that some or all of the syncing work is done in the cloud, or perhaps the syncing is done locally but requires a significant enough buffer that local content can't be synced.

Amazon has a huge budget for R and D though, so I would not be surprised to see the capabilities change in time. I bet they could do it without infringing on patents as well, or perhaps can work out a deal with Sonos. I don't think the market is at a point where the differences between what Sonos and Amazon (and some of the others) do in multi-room has enough value by itself to sway a consumer's purchasing decision. May be some day it will, and may be Amazon, or others, will have improved their system by then.

Hey Kumar, small point (and apologies if you already know this) but you can stream local music to an echo with Plex.
No, I did not know this, so thank you. Something I need to investigate.

I don't think you can stereo pair echo dots,
Amazon says you can:
https://www.amazon.com/gp/help/customer/display.html?nodeId=GPSQQS7N3SBKAEWU
The 3rd Gen Dots are supported devices.

Danny - I think you are right that multiroom with Echo devices is not possible when the source is via Bluetooth. Or rather,my attempts to do so failed, which is not at all the same thing!

To the Plex suggestion: my WD NAS works fine for Sonos, but it seems that it is too rudimentary to run Plex.
How does it work though if it does? How can one play playlists via Alexa/Plex from music files on the NAS - the Sonos ones rest in the Sonos boxes, so is there some way different ones can be made for this purpose?

Digressing a little:
A play 1 pair + Sub remains a reference system for me for music play sound quality in a up to moderately sized space.
I wonder how a Dot pair + Echo Sub, at less than half the price/footprint of Sonos, compares today, if one only needed streamed music as a source. Even if it isn't there yet as is very likely, it may be within striking distance after a couple of more iterations in a couple of years - because I don't see Sonos moving the bar on sound quality much further on the music front. Though I suspect that adding bluetooth capability to that configuration will take more than just iterations, but that isn't a lack for those that do only streaming; and I am sure that even today, the Amazon cast feature works for those that don't like talking to their music system.

Back on topic, it would be interesting, as an academic curiosity given the silly pricing, to know the patents situation surrounding the recently launched B&W Formation line. From their site it looks like it does all that Sonos does using some kind of mesh wireless, but also does Hi Res and Bluetooth.

Some more info:
https://www.jdsupra.com/legalnews/audio-giant-silencing-the-competition-72890/