Why have you added *.m4a suffix? Was that not in situ (and perhaps hidden from view) when the tracks were encoded/transcoded to the Apple AAC compressed format anyway? Did you convert them and if so what format were they originally? - Do they still each play in your iTunes App, or whatever  suicidal player App you are using?

What protocol are you using to share the local library folders (note it now needs to be SMBv2, or higher). Sonos no longer supports HTTP/SMBv1 library shares.

Here’s two Sonos Support links and a YouTube video aswell, that perhaps may assist you to setup Sonos access to the share but some things may be irrelevant depending on the device you are using for the SMBv2 (or higher) share…

https://support.sonos.com/en-gb/article/share-your-macos-music-folder-with-sonos

https://support.sonos.com/en-gb/article/add-your-music-library-to-sonos

https://youtu.be/kjhKhDuvgDY

Thank you for your response. Please know that I did NOT add any suffix - it has just mysteriously appeared. And I apologize in advance that, as a senior, your advice is beyond my comprehension. I wonder if any community members are able to articulate a possible solution in plain language. 

Did you read the links and/or watch the YouTube video? Were they not able to get you started? If not, you maybe best to speak with Sonos Customer Support to see if they can assist you. Here is the link if you decide to perhaps give them a call…

https://support.sonos.com/s/contact

Thank you. I read the links (unhelpful jargon) and have watched the video.  I will try customer support.

https://support.sonos.com/en-us/article/supported-audio-formats-for-sonos-music-library

 According to this link Sonos only supports m4a up to 320kbps.  M4a is an Apple extension for an Apple audio file.  Not sure if the 320kbps restriction still applies.  I believe m4a is the file extension for AAC ( Advanced Audio Codec ) which Apple chose to use for there lossy files as opposed to MP3.  AAC does support up to 512kbps.  See if you can find what the bit rate is of one of your unplayable files.  Apple downloads used to be 256kbps, but not sure if that is still the case.  I personally have no AAC files to test. 
 

 Further reading:  the extension m4a can also be ALAC ( Apple Lossless Audio Codec ) Apple’s version of FLAC which is also supported by Sonos according to the above link.

I did not add the m4a suffix. Somehow it just appeared on every title whether downloaded from Apple or uploaded from my CD collection. 

It’s more likely going to be the library sharing protocol - I’d speak with Sonos Customer Support to see if they can help walk you through the setup. If using Windows 10 or 11 then there is a step-by-step walkthrough here with some screenshots posted by another user…

https://en.community.sonos.com/controllers%2Dand%2Dmusic%2Dservices%2D229131/adding%2Da%2Dwindows%2Dsmb%2Dshare%2Dstep%2Dby%2Dstep%2D6892060

I using a MacBook Pro

Then the earlier YouTube video link shows you what you need to do … I recommend though that you add the library to Sonos using the mobile App… just so you don’t use the first two ‘add library’ options in the Desktop App, which use the HTTP protocol.

 All files have extensions.  Sometimes they are hidden.