I have the same issue whether I am in the home network or outside (1 Gbit).

When outside your home are you also streaming to a Sonos speaker? If not this would point to an issue with your NAS or network.

When was the last time you restarted your network/router and NAS?

Have you considered assigning static IPs to your speakers and NAS if you have not done so already?

With certain songs makes me think that could be a wrong/unsupported codec for Sonos speakers.

Really frustrating to troubleshoot issues like this with the limited information Sonos provides us on internal operations. Best to recreate the issue, submit a diagnostic and call Sonos support where they can see the internal logs.

This is really frustrating.
And when you contact SONOS support, they say you have to contact PLEX support. They say the service does not come from them.

Plex transcodes unsupported formats and resolutions to supported AAC so this should be a non issue. 

Well they are not wrong, it does come from Plex.

As part of trying everything, did you try changing the track's file name and meta data?

The easy way is to make a copy, call it test1, then clear out all Tag information and any embedded image data.

This fixed my (long ago) issues with unplayable tracks. Then putting back the removed data one item at a time identified the problem data item, editing that identified the exact issue. 

Why would this be a problem exactly? Surely there is a way to systemically fix it? 

Are there best practices anywhere? 

I have my Plex on Truenas core. Mine is intermittent, some songs play, then get the above error like the OP. 

How to best test if things are not optimal? 

Fixing the underlying issue is certainly an option but so far it hasn't happened. Don't know if it is a Sonos or Samba issue, neither user fixable.

Working around it with either a Samba configuration file change or a mass-edit with a tag editor is something a user can do.