Why is Sonos restricted to connecting to the 2.4 Mhz band?

I would assume it has mostly to do with compatibility for all of the Sonos devices that Sonos has had in the market place, some of which were sold long before the 5Ghz chips were cheap enough to be included.

Oh, there's another reason, too. 2.4Ghz has much greater distance characteristics, and less absorption by materials. So it reaches around your home much more easily than a 5Ghz signal would.

I've been enjoying Sonos since 2011. My understanding is that all current Sonos products are designed to run on the 2.4 Ghz band. The exception to this are the controllers (laptop, iPad, iPhone etc) which can run on the 2.4 or 5 Ghz band. That said, I googled the phrase: "Are all Sonos products currently restricted to running on the 2.4 Ghz Wifi band?". Looks like this has been debated going back at least 4 years. As good/great as Sonos products are, I believe an option to switch bands would be a huge enhancement so everyone could choose what works best in their environment. The question remains: Can it be done? As one example, I have an Apple TV that I switched from 2.4 to 5 Ghz. My TV/Apple TV/Playbar/Surround system is located far from my router, as are some of the Sonos components. Switching to 5 Ghz made a noticeable difference in speed, with very short loading times for videos and no loss in streaming due to a dropped transmission signal. And more fun to watch!

A 2.4 to 5 GHz radio conversion could be done, I've done things like that (legally) for systems I had the design and software for.

Internally on Sonos you'd have to crack open all of your Sonos devices that didn't have a WiFi card that works on 5 GHz and replace the 2.4 GHz WiFi card, then patch the Sonos operating system and application software to be aware of the new card's abilities and to use it. Neither should be impossible to do but it would be far from easy, you'd have to plan on continuing to patch new software releases as they come out or potentially have some options stop working.

Some help could be gotten by starting with the Sonos released GPL code but you'd have to reverse engineer the unpublished Sonos applications code yourself. Doing anything openly to the non-GPL code would probably trigger the DMCA so you'd really be on your own doing it and it would be risky to tell anyone once you did. On older gear you'd probably have to give up some functions to make space in their limited memory for the new 5 GHz options too. Hardware modifications might be possible to reduce the limitations since you are already modifying the core code.

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If I really had to have 5 GHZ support I would look at adding an external network access point to each Sonos device, connected through the Ethernet port. That avoids most of the above technical problems above and all the legal ones. You likely could do a test design with a couple Raspberry Pi computers, the 3B version has 2.4 and 5 GHz wifi as well as a 100 MBit Ethernet port and should be able to support a Sonos stream or two without bogging down. Once you have it working on the cheap Pi computers you could look for more capable hardware, both WiFi range and CPU horsepower for your final design. Maybe some of he Ubiquity Unifi long range gear for the WiFi side and your choice of system boards.

Since this could all be done externally to Sonos's gear the DMCA, patents and the rest wouldn't be a roadblock and you could make it a group/public project if other folks were interested. Github or Sourceforge would make good public hosting spots.

Not mentioned here is the fact that they reserve the 5gHz wifi band for surround links, which would get contaminated if you also run the Sonos mesh on that band.

However, that could probably be resolved with a somewhat smarter topology management (today players try to connect directly to the appropriated root node always, except when they act as surround), which could also improve stereo pairs as well (since they would be much better off connecting to their master node).

But with all changes you will always get a group of people that gets a worse experience, which you need to consider as well. I feel the situation has improve somewhat though, with people switching more and more to 5GHz for other equipment and not polluting the 2.4 as much. And the opposite, that the 5GHz band is starting to get cramped, and with new equipment using 80MHz bands, it's as bad as the 2.4GHz band was (only leaving 3-4 channels fully separated).

However, that could probably be resolved with a somewhat smarter topology management (today players try to connect directly to the appropriated root node always, except when they act as surround), which could also improve stereo pairs as well (since they would be much better off connecting to their master node).
This actually happens today. In groups (a pair is just a special type of group) the system will try and use Direct Routing between nodes for the stream, effectively bypassing the spanning tree topology. This also works in Station/WiFi/Standard mode, which is why Sonos recommend that where there are multiple APs they share the same channel.

The only advantage to the 5 GHz band in my opinion is the number of channels (24?) offered by the newest hardware compared to the 3 clear ones on 2.4 GHz.

Should be easy enough to pick a 5 GHz one Sonos isn't using for surround communications.

A key point about 5GHz is the use of per-frame bandwidth management in AC. Despite nominally occupying 80MHz -- and therefore all the available 'low' channels (36-48) at an 80MHz width -- it's able to avoid clashes with N transmissions, and indeed with other AC signals.

My TV/Apple TV/Playbar/Surround system is located far from my router, as are some of the Sonos components.But Sonos was designed to run on the SonosNet mesh, making the distance from router to component a non-issue in most cases.

In groups (a pair is just a special type of group) the system will try and use Direct Routing between nodes for the stream, effectively bypassing the spanning tree topology. This also works in Station/WiFi/Standard mode, which is why Sonos recommend that where there are multiple APs they share the same channel.

Do you have any reference for that? The topology is basically on ethernet level, not sure that they can actually control the dataflow from the app-level...