buffering issue

Not really, they don’t publish much on their update notes, either for the controller, or the firmware on the speakers, in terms of detail. Release notes are here for the controller, and here for firmware. Note that the dates in these links are US (month/day/year).

Honestly, I’d try a bunch of things that you’ve already tried, as each ‘change’ of the controller, the router’s system, or the speaker’s firmware could potentially bring issues.

In the likely order I’d try:

1. Check for updates. Both from Sonos (in the controller) and from TP-Link for the Deco router(s) 
2. Reset the network, by unplugging/powering off (for battery powered speakers) all devices…anything that says Sonos on it, including BRIDGEs or BOOSTs. Reboot the TP-Link Deco devices, wait two minutes, then plug in (or power up for battery powered) all Sonos devices. Wait two more minutes, then test. Note: if this ‘works’, if you can, assign reserved IP addresses to all Sonos. 
3. Remove the Internet cable from the Sonos. You want the Sonos on your network, not trying both your network and SonosNet where available. It will reduce bandwidth potential conflict.
4. Finally, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. Don’t post the resulting diagnostic number here.

   There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution. And you may get a more seasoned representative than your initial interactions.

   When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

Thanks for the response.  
I deleted the Ethernet cable.  
I have rebooted the speakers and the mesh network multiple times. 
I have tried submitting a diagnostic several times over the past few weeks, and will try again, but that has not been helpful so far.  

I decided to try Sonos Radio.  That had no buffering, but way too many ads, and it is a non starter for me to buy premium hardware only to listen to ads.  Yeah, I could buy the ad free Sonos Radio option, but why would I want to add another revenue stream to them just to get what should be a free service through my self built playlists on Apple.  (3500 songs & growing).  

I will not abandon Apple Music b/c all my devices are IOS, and Bluetooth to my vehicles and Sonos Bluetooth capable speakers on my boat all work flawlessly.  

 I tried Amazon Music.  That’s been running for about 90 minutes now with no buffering.  Does Bezos own some share of Sonos? Again I lose access to playlists I have built over time I don’t want to lose.  Where else can I get John Lithgow and Bebe Neuworth singing Baby It’s Cold Outside?  
 

so, now I narrowed it down to some conflict between Sonos & Apple.  Any other suggestions? 

Just to see, try powering down the two additional mesh nodes and see if the Sonos situation improves.

Are you set to 20 mHz channel width on 2.4 gHz, on 5 gHz 20, 40 or 80 all seem to work well here.

Check your three 2.4 gHz channels and use the one with the lowest utilization. Same for the 5 gHz channels. 

If only with Apple Music I suggest:

1. Sign out of Apple Music
2. Delete Apple Music from Sonos
3. Update Sonos app if needed
4. Sign into Apple Music
5. Add Apple Music to Sonos
6. Test for buffering

Also, I’m not a fan of Band steering such as locking Sonos to one band. Sonos can operate on 2.4Ghz and 5Ghz. Speakers will switch bands as needed to maintain a communication as WiFi signals go strong and weak throughout the day. Sonos will attach to the strongest signal without missing a beat. Especially so in a mesh network which is the reason to have a mesh.

Also, I recommend using the same SSID for both bands.

Additionally, turn off Airtime Fairness.

In my opinion ISP’s don’t know “jack” about Sonos. Sonos is not a simple WiFi capable device as they are constantly talking between the router and themselves. ISP’s think only of a single device needing a connection not constantly sending and receiving packets to maintain a sync for music.

Thank you for these suggestions.  
I did look into the Sonos update stuff.  I am up to date on updates, running 85.00.31.  
Stanley, each time I have tried rebooting, I have powered down 3 nodes and 8 speakers to no avail.  
AjTrek1, I haven’t deleted the Apple Music app, then reinstalled it.   I will try that tomorrow.  Past my bedtime now.  Thanks for the idea, I will let you know how I make out,  if I do that, will I have to delete & reinstall on all of my IOS devices?  Likely yes, I guess.  
 

again, thank you all.  

Stanley, each time I have tried rebooting, I have powered down 3 nodes and 8 speakers to no avail.  

Powering down all three nodes leaves you with no Wi-Fi, powering down only two leaves your Wi-Fi available but without the complications of the mesh.

Leave your Sonos powered up, leave the main hub powered up, power down the remaining two mesh units, try using the Sonos.

Just a small correction for future reference. Since the Sonos Bridge or its successor the Boost are network components, you probably owned a Connect, that indeed ‘connects’ non-Sonos hardware to the Sonos ecosystem.

Thank you again.  
The first node on my mesh network is physically under the other two.  It’s in the basement because that’s where my internet feed comes in.  We don’t have a router, we are in a Fiber area, an Ethernet cable comes in directly to the first node.  
I tried playing a playlist I have on Apple to those two SL speakers, they are also in the basement.  I powered down the secondary nodes on the main floor and grouped just the two SL speakers in the basement to test.  
it’s tried a half dozen sings so far, every one skipped. Some in the first line, others further into the song. When it skips, I sometimes see ‘Cannot play XXX it is not encoded correctly’. XXX= song name.  The songs named as improperly encoded played fine before this fiasco started.  It’s been going on long enough for me to remember some songs that were ‘improper’ played and skipped later.  
I switched to Amazon Music and it plays fine.  

106ralley.  You are right, I had a connect.  It has been offline since before this issue started.  I had hoped it would make a couple of the Sonos speakers unnecessary, but that didn’t work.  Deciding to delete the connect lead to buying the two versions of Move. 

With Amazon working and Apple not it sounds like the local network isn't the issue, more likely the connection to Apple.

Get the issue, submit a diagnostic and note the number, call in and talk to support to have them look at the diagnostic that has data we users can't see.