Here are the local library track formats supported on Sonos, perhaps ensure they are compliant with those… 

https://support.sonos.com/s/article/79

Check any embedded folder art, or tags, to ensure thats not part of the issue.

Try converting one or two sample tracks, using a different encoder, just as a test? 

Oh and don’t forget to reindex in the App, after changing things before re-testing the changes.