The controller App software needs to be in sync with the firmware installed on the speakers. As the speakers have already been updated to the latest firmware, there is no way (that I’m aware of) to downgrade the device firmware to allow the old controller software to be used. 

How is the speaker firmware updated? Does it happen automatically when any connected controller app (on any platform) is updated? The system in question is a single Playbar, controlled thus far from an iPhone and another Mac (controller versions unknown right now but I would imagine the iPhone is up to date)

This can happen automatically, or manually - it depends if you have ‘automatic updates’ enabled/scheduled in the Sonos App ‘Settings/System/System Updates’, or you can of course manually check/install the updates yourself.

An update, first checks for updated controller software and when installed, a second check may install the firmware (if not automatically installed already). The App will usually notify you if there’s a controller App update available.

The two types of update are entirely separate and so you may get a firmware update to a device or devices, with/without a controller App update, or vice versa. 

Understood. The Playbar is confirmed to be running 15.2, which means I need a 15.2 controller, and the macOS 15.2 controller is unsupported on macOS 10.11, so on the face of it I am out of luck.

However, the 15.2 release notes say this: “The Sonos app for macOS will no longer receive software updates on macOS 10.11. The app can still be used to control commonly-used features”.

What does this mean? Version 15.2 should in fact run (for commonly used features) on macOS 10.11? 

Or the previous version (15.1.2) will still work even if your speaker has been updated to 15.2

Or 15.1.2 will continue to work but only if your speaker is never updated?

Sonos tend to fully support a controller device fully up-to 12 months (approx.) after the manufacturer has ended its own support for its operating system and then it becomes partly supported for music playback/control, but has no admin. features for setup/install of Sonos devices etc.

A list of all the current unsupported operating systems are shown here…

https://support.sonos.com/en-us/article/using-sonos-with-unsupported-operating-systems

It shows that MacOS 10.11 is still partially supported. So I would just try uninstalling the current App fully (important step) and then reinstall the latest App and on opening, choose to connect it back to your ‘existing’ Sonos system on the network and see if that works for you.

In some instances I’ve seen some users say that rather than uninstalling the App, that these steps (below) have also worked for them with MacOS...

Authorise the “Sonos Installer” under System Settings → Privacy & Security → App Management. Once allowed, you should be able to use auto update from the Sonos App itself.

Returning to this after gaining access to the iMac in question, I can report that uninstalling, changing the Privacy setting, etc. all fail to help. The iMac had been set up with a new hard disk, a fresh install of the system, and the Sonos app was one of the first 3rd party apps to be installed anyway.

Unfortunately it looks like Sonos haven’t tested the 15.2 app on MacOS 10.11 at all. Despite the app’s LSMinimumSystemVersion being set to to 10.11, the reason it crashes is that it has a dependency on a framework built for MacOS 10.13.

So it’s informing the OS that it can run, but it cannot. In case anyone from Sonos is interested the framework in question is SCDependencyKitMDCR, and I’ve verified its LSMinimumSystemVersion is 10.13. Crash log is here: https://www.dropbox.com/s/11tjn2i226ne7nt/Sonos_2023-04-15-111106_Kates-iMac.crash?dl=0

I totally get that El Capitan is old. Upon the most recent update, I called in and spoke with someone at Sonos and asked them for a link to the last working version of the S2 software. They did provide me with a link and I was able to install that version and simply ignore the requests to update the software. But one day, the whole thing crashed and installing the version shared with me asks to update the software.