IP reservation is often recommended as it helps to avoid trouble from lousy routers, especially when Sonos units reboot during a firmware upgrade. Perhaps you were encountering IP conflicts for such reasons.

Your community profile doesn’t mention what country you’re in but, as you’re no doubt aware, in many regions your choice of 5GHz channel means that it’s subject to TPC and/or DFS restrictions. Best avoided IMHO.

Yes, great additional tip to avoid DFS Channels on the 5GHz radio with a product as sensitive as this is! I forgot to take this into consideration when fine tuning my setup to best support IOT devices such as these speakers, as there is no real reason to use DFS in the rural quiet (RF wise) environment where I live. 

After playing fine for 2 hours in testing after I perceived my setup as stable.  1 of the speakers dropped off the app and grouping of 3 speakers after 2 hours of flawless playback with all 3 speakers at once was the best I’ve seen in 4 weeks until it broke. It was my lone 5GHz connected One SL speaker that dropped off.

I am not certain why, could be DFS channel related as I was running on channel 132 at the time during this test.  Strange why it would bump the speaker off after working for multiple hours, another great question for a Sonos developer as it clearly was working on a DFS channel so its supported by the hardware to some degree, I had no other broadcasting Wifi devices running at the time. This speaker is 10ft from the AP, and not next to any other RF emitting devices within a 15ft radius.  Perhaps the 5GHz radio/chipset in these speakers is low quality, as the wireless signal on this speaker on 5GHz was roughly 45-50 SNR or 40-45dbm during this test. 

Any case I turned all DFS channels off, that’s channels 50 through 144 here in the USA.  After rebooting my Wireless AP, 2 of the speakers now connected on 5GHz (one of them on a weak signal of only 20 SNR or -70dbm).  Played for an hour or so before the 1 speaker with the weak 5GHz signal disappeared from the App and the playing group, other 5GHz connected speaker with great signal strength was fine.  Checked my Wireless AP, and sure enough that speaker that dropped off the App and the group switched radio bands to 2.4GHz, as the signal got too low.  The Sonos App is unable to recover from this behavior of simply switching bands without a complete reload of the speaker, Wireless AP and phone (in my experiences so far).  

I decided to build a new Wireless SSID just for the Sonos speakers and only to operate it on 2.4GHz, as it seems most stable for these speakers (prevents speakers from switching radio bands by locking it down to a single radio band), which is ironic as generally in the WiFi industry we are pushing 5GHz as much as possible for many reasons.  Ideally I would want to build this Wireless SSID for these speakers only on 5GHz, but the 5GHz coverage in my house is not adequate enough for all the speaker locations. 

I may also consider using the “SonosNet” aka private wireless mesh network with a single wired speaker to see if its anymore stable.  Based on the channel selections available in SonosNet, I chuckled when it says its proprietary technology and see it gives you channels 1, 6, 11 to choose to operate on….  those are all 3 of the valid 2.4GHz channels in 802.11 Wifi. Kind of neat idea from a high level, but technically poorly implemented as the channel has to be statically set by the uninformed consumer/end user.  Its not smart enough to choose the best channel on its own, or give the consumer/end user the ability to either tell the speaker to auto select the best channel OR statically set it yourself. If you have a smart enough Wireless router, you can tell your Wireless router to auto-select the best channel to avoid interfering with the SonosNet private wireless mesh network feature, otherwise you have to periodically keep checking the RF environment around you and see which 2.4 GHZ channel is best to operate on for SonosNet.

DFS is of course Dynamic (the D) hence the AP is obliged to listen out for priority users and vacate the channel if required. The option of 5GHz for the WiFi (“wireless”) Sonos operating mode is relatively new, so perhaps the units have yet to handle having the rug pulled out from under them, by an AP channel switch, as seamlessly as we would like. Reconnection can sometimes trigger a soft restart.

My understanding is that, within certain RSSI limits, a unit will prefer 5GHz to 2.4GHz but below a threshold it will revert to 2.4GHz.

Apropos your chuckling about SonosNet, it’s perhaps before your time so you may not be aware that when Sonos first debuted in 2005 SonosNet was the only setup mode. Given what was available at the time in the WiFi world it was something of pioneer, offering an extensible mesh and separation of time-sensitive audio from other, bursty traffic. It’s still the best operating mode for larger systems. 

Had you set up your system from new in SonosNet mode it would have chosen the best of 1/6/11 considering the environment. Yes, it’s fixed after that unless the user changes it, and this is most definitely welcome, in the interests of maintaining mesh stability.

Routers auto-select channels out of the box and, as with their all too frequent use of 40MHz at 2.4GHz, often get things completely wrong. They regularly don’t detect and avoid SonosNet. The standard advice here is to fix WiFi out of the way of SonosNet.

Another point that you may not be aware of is that Sonos units perform best in groups when they share a common channel, since the stream flows can then be sent directly between wireless peers. This also applies to bonded stereo pairs and non-HT Subs, which are a special type of group. 

Deleted in interest of civility.

Well I feel a little better that it’s not just me. If “Stability Tips from a Wifi Engineer“ can’t figure it out. 
My issue is simple, my Sonos one drops and skips when trying to play Spotify from my iPhone. 

It isn't as complex as it is all being made out to be.

I had a 5 zone system with just one unit wired to the router, and reserved IP addresses for all Sonos units including the phone that hosted the Sonos app. I never had any significant issues using Sonosnet once it was automatically set up by the system, keeping Sonosnet on channel 1 and the home WiFI on channel 11.

I now have more Sonos units wired back to the router, but that is for the special case of needing TV audio piped around to these using very small buffer sizes to avoid lip sync issues.

And I am not an engineer, let alone of the WiFi kind.