Answer 1: Sonos’ data is stored on the speakers, each and all of them. 

Answer 2 is not clear. Were they all able to be seen while on the NAS? Instinct suggests that they don’t match the necessary formats, but if they were seen while on the NAS, then I’m wrong on that guess. 
 

Have you called Sonos Support directly to discuss it. They might be interested in some particulars, too. 

Thanks for your response.  All of the songs are WMA format.  However, you brought up something I was not aware of and that is very interesting.  I checked the bitrate of the songs that came off one of the albums I am having a problem with and the bitrate on those songs was crazy high - over 800kbit/s.  I decided to convert the songs to a more appropriate bit rate(320) and that did the trick.  Sonos recognized the artist and the songs I converted.  

Well done, thank you.

Additional information - I checked another one of the albums that Sonos didn’t “see” and found the songs were 256kbit/s.  I converted these to 320 and now Sonos sees that album also.

Another win.  So, not just high bit rate but low also maybe?!

List of formats shows “WMA up to 320kbps” but doesn’t mention lower as an issue.

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

What happens when you ‘convert’ one, as a test? I was unaware of a lower limit as well, but could easily see either a ‘lower’ limit, or a non-standard (not a power of two, for instance) conformity being an issue. 

I’m not so sure of the lower limit as an issue so much as I am wondering if it’s just the file getting modified in some way that is the actual resolution for the lower bitrate songs that weren’t showing up in Sonos.  I changed another album where the songs were at 256kbps and it appeared in Sonos also. That’s 3 albums that I have modified the bit rate on and they showed up in Sonos when before they didn’t.    Yet, I have other songs at the lower bitrate that show up in Sonos, no problem.  I think I’m leaning more towards the lower bitrate songs being modified as the resolution?

I modified the bitrate on a single song in an album that didn’t show up and that song and artist showed up but not the rest of the album until I modified them also.

what ya think?

Don’t know, sorry. Your test suggests there is something else going on, but that’s speculation. 
 

I’d be tempted to try opening one that isn’t being read currently, then do a ‘save as’ and not change anything but the name. That would suggest to me that the original ‘rip’ was ‘wrong’ in some way, although how, I have no idea. Bad characters in the metadata? It really is perplexing, if that indeed works. On the other hand, you have a potential solution. Years ago, I used mp3tag (haven’t opened it in a long time) to modify multiple files at the same time, it may be worth looking if that’s a possibility for your situation, so you don’t have to do each file individually.

I continue to be interested, although at this point I’m not sure I’m really helping.