HI.  I have no idea what happened, but I do know that Sonos has no capability to modify files stored anywhere in any way.  This may have been coincidental or something else you did inadvertently.

The files are certainly not damaged—they would always play in the Mac click-to-preview-without-opening-an-app thing. But the permissions were changed: all audio files except those sitting on the desktop show “SonosDMS...Custom” at the top of their permission list. I read that that doesn’t matter, but I don’t know. And yes could all be a coincidence, but nothing remotely like this has ever happened on this laptop before and hard to think of anything unusual I did. 

But as @John B suggests, Sonos doesn’t modify in any way the files, it’s strictly a ‘read only’ application. You tell the Sonos software where to look for files, it then ‘reads’ what it can, without ever modifying anything. 

@Andrew Flounders Off topic, but I wondered why you felt the speaker was not right for you?  If you will excuse my asking as it's none of my business really. 

To start with the set-up was a bit off-putting. If I paused a file or stream it would run when I tried to resume but with no sound. To get sound I would have to switch to inbuilt speaker and then back to Sonos and it would be on. Or the laptop would just drop connection with the speaker entirely.

The Sonos help-line was very good and they managed to fix that last problem, mostly. But playing was still spotty: I would start a file and the sound would come on 5 or 10 seconds late: if I pulled the cursor back to the beginning the sound would play then.

I tried using several methods to start-stop — the controls of the Mac apps, the Sonos laptop and iphone apps, the buttons on the speaker, a third party controller for Sonos from the Apple App store, but it became evident that they were not properly integrated with each other. If I pushed the button on the device the audio would stop at once (refreshing punctuality!) but could neither be restarted there not via the computer.

I got tired of having to spend so much time getting to things to work (via the helpline, talking to the salesperson in the shop, trawling through entries in these pages) and of feeling like I had to coax it into behaving.

Ultimately though, the thing that made my mind up against is the lag in the speaker responding to the controls on the laptop. I do a lot of playing along to too-hard-for-me music with refrequent stopping and starting and I need that to be instant and clean. As I understand it, Sonos requires a degree of lag to manage arrays of speakers (something I have no need for).

I thought about getting an Apple Homepod 2 instead on the assumption that it would manage things more seamlessly, but I think I probably just need a speaker created in 1973. On my side I guess I didn’t really understand what a smart speaker is for and the Sonos webpages, unsurprisingly, gave a rather optimistic account of how their speaker was going to work via a Mac laptop. These Sonos community pages contain a lot of useful information exactly relevant to my problems, but the only way I was going to find that information was by buying the Sonos speaker and encountering those problems to then search for. 

As for my current problem with misbehaving audio files, it’s hard to believe that it has nothing to do with the Sonos speaker, if not via its arrival and operation then via its deletion: maybe the cause of the problem lies with the third party controller or the application that handled the removal. From my perspective it doesn’t make much difference. The Sonos Era 100 sounds very nice and is unobtrusive and elegant, but I think I’ll shift to an ugly box speaker that plugs in via a tangle of cables but is a simple and obedient extention of the laptop. 

Now to revert my system back a week...

Hi.  It’s a done deal now - or rather an undone deal - so I won’t try to persuade you otherwise.  Indeed, you may well be right that it isn’t the best speaker for your needs.  Sonos is designed as a networked mulriroom system, not as a computer speaker.  Having said that, the Era 100 does have Bluetooth and Airplay so is equipped to be used in many ways as a laptop speaker.

It isn’t clear to me what means you were using to connect to your speaker-  network, Bluetooth or Airplay.  I suspect it isn’t clear to you, either, but that may be unfair.

It may be that your best solution for your needs is just a Bluetooth speaker.

The Sonos app is just a remote control for the system when used in network mode.  It does not copy or manipulate files.  It is irrelevant when using Airplay or Bluetooth.

I hope you find something that better meets your needs.