I couldn’t find anything affordable. Ended up just unplugging other electronics, to include things that did not have a radio, until I found the problem.

It was an WD external hard drive, no WiFi radio, just garbage design.

I have had issues with an old microwave, and a new neighbor’s router strength, location, and channel in use. Back in the day, I had issues with sunspots, I think, but am not sure, the interference they generate is particularly hard to pinpoint. I’ve had a friend with issues being too close to power lines (not in his house, but that potential is there, too). 

I’m not aware of any device that would show all the interference from different devices. 

What I do use is a Wi-Fi analyzer app on Android. If you have Android, there are many free apps. E.g.

https://play.google.com/store/apps/details?id=com.vrem.wifianalyzer&hl=en

That will show the networks it hears and shows their channels. Recently my neighbor changed, and the new neighbor’s access point was on the same channel as mine causing interference. If you already haven’t checked with an app like that, I’d check just in case if there are new Wi-Fi networks around operating on same or nearby channel.

In apartment buildings where people can have many Wi-Fi networks the networks can be on same channels causing interference (obviously depends on the building’s walls etc.). House owners have it better.

In my router I change the 2.4 GHz channel width to 20 MHz in order to try to counter possible interference. 

These are Sonos recommendations but I’d say it depends on what devices you have and what works best in the house/apartment layout (e.g. 5 GHz is too unreliable at my place so have Sonos devices on 2.4 Ghz):   

https://support.sonos.com/en/article/recommended-settings-for-using-sonos-on-wireless-networks

Talked to a colleague who demoed Aaronia Spectran V6 device: Compact USB Real-Time Spectrum Analyzer - SPECTRAN V6 PLUS 250XA-6 - Aaronia AG. Affordable is questionable as it costs a few thousand euros/dollars. It shows activity also on the 2.4 GHz and you’ll see activity from access points, mouse, keyboard, etc. devices operating on that band. If money is no object then I’d try to source that. Could be a bit of an overkill for this situation, though. To use that you’d turn off any possible devices one by one and you’d see if their signal disappears from the view.

For Android they recommended Ubiquiti’s WiFiman https://play.google.com/store/apps/details?id=com.ubnt.usurvey&hl=en 

That won’t show microwave ovens, mouses etc. but shows the Wi-Fi networks nicely. You'd see if there are any operating on same/nearby channels possibly causing interference. 

https://support.sonos.com/en/article/reduce-wireless-interference

If you need to resort to a general purpose spectrum analyzer, look into a short term rental.

Are there any nearby baby monitors or Zigbee devices?

The irony here is that 20 years ago when Sonos built its reputation for rock solid music synchronisation of music across your home, network speeds were a lot slower but back then no one spoke about having to check your network.

I was introduced to Sonos by my brother in law, one of the least IT savvy people I know, it just worked for him “out of the box”. 

We have to question why, given all the improvements in network technology, Sonos performs worse today than it did years ago. 

A few things have changed over the years. Back then a “networked” home might have had a couple computers and phones, a printer, and whatever SONOS. Now we will have more computers and phones, and many more WiFi devices, such as cameras, doorbells, thermostats, switched outlets, kitchen appliances, air conditioners, etc. Older networking equipment may no longer have the capacity to support SONOS along with the other items. SONOS requires more robust network support than most consumer items. SONOS units must be able to chat with other SONOS units and the controllers very rapidly. If a transaction between a refrigerator is delayed by other network traffic, even for a few minutes, it’s not a big deal, however, delaying an audio sample for a millisecond is catastrophic. Sonos buffers data for 75ms or more. I’ve observed SONOS players finishing a track for a minute or two after the network connection was removed. This is very robust network support, however it has limits. If network support is so poor that a buffer cannot be populated, short, additional network spasms can result in a music mute even though the system seemed to be playing OK (but was on the edge).

In many cases updating the network support can help.

Another issue is that the recently updated SONOS system uses the network differently. One can argue that the new support will be better long term. Unfortunately, the default settings of routers may not support features that the updated SONOS requires for reliable operation.