Perhaps it’s something related to the embedded Cover Art image resolution. See this support document for the Art requirements: 

https://support.sonos.com/s/article/543

Thanks for the reply, but both resolution, dpi and file size are within Sonos requirements.
Very strange

I can’t assist, but the case it shows the same for the Walkman seems to infer there is an issue with the embedded Art to begin with, I guess … but agree it’s a strange one.‍♂️

Have you tried saving the art in the album folder, as described in the link?

if you do so you may have to remove the embedded art from the files.

Thanks for the suggestions!

I like to have the cover art embedded into the file instead of separate in a folder.

Converting the file into .iff and back to .jpg does work, but I was surprised that I need to do that for each download for the last 1~2 years. So, was wondering if more people encountered this issue.

My album art is both embedded in each track and stored as a .jpg in each Album folder, but not sure which art gets chosen by Sonos, presumably it’s folder.jpg? - in my case both are the same Art image anyway and set to just 600x600 pixels. I run all my purchased/ripped music tracks through mp3Tag software and that seems to quickly sort my tagging needs and the Art. 

The reason I store the cover art in each of those places, is that over the years, I’ve seen some tracks lose their embedded art, or the file in the album folder gets overwritten - I’m not sure why or by what, not by Sonos as far as I can tell (it’s read-only), but quite possibly by other Apps .. it’s one of life’s mysteries in my own case, but now and again I go through the library and give the tracks a bit of a tidy up.

hi @biggetjeindewei if you buy tracks with iTunes or Apple Music then the cover art is not stored in the metadata of the music track - this is why I always copy paste a jpg file again in the Artwork tab of a music track to make sure it is stored in the metadata