Try to clear the cache of the Sonos app also you can view the guide of how to do this :)

Have you forced a re-scan of your local library?

Do you mean update Music library? At first it would not update saying the pathway had been lost even though music still played. It will now update but makes no difference.

on one track it shows 07 Filthy_Gorgeous.m4a but the track will not play,  some others showing in the format do play.

leo07, I am 73, not very computer literate so did not quite understand, what is the cache of the Sonos app?

Thanks for your comments by the way

Oh my bad, the cache of an app is the space used in the device that contains much information that the app requieres (it could be like the cover images, in this case like your system and their names) and often it doesn't refresh so it could be old data of some things.

But now I don't think this could be the problem so if you can tell us how did you converted the .m4a files into .mp3.

Thank you for all the information :)

I duplicated all the tracks in I tunes so that I have the originals and an mp3 copy in Itunes.

I then used a program to duplicate the mp3 and move them into a separate file and then changed the Sonos pathway to lead to this file only.

I thought that the old non mp3 tracks on I tunes might somehow be interfering in some way so experimented by deleting some mp4a tracks on my I tunes and just leaving the mp3 tracks, I then updated but it made no difference. 

Try to view if there's metadata (information embedded in the songs, such as the cover image, album, artist/s, etc.) with mp3tag in Windows and if there is clear it, this could solve your problem.