Hi

If I understand your question...the line-in on an Era 100 works the same as a line-in on a Sonos Play 5 (gen1 and gen2) and on the Sonos Five.

Yes! That’s right.  Could I use any MP3 player?  I’ve been using strictly apple products hence the iPod Classic. I found this configuration easier and more reliable than NAS drive. Could an iPod touch be used?  

Thank you!

Any device that creates a line level output will work. Slightly more problematic are variable outputs, you end up with fighting volume controls. Usually this occurs if you’re using a headphone jack to feed the Sonos, I usually suggest starting with at least setting the volume to 50% loudness, to ensure there is enough signal output for the Sonos to latch on to. 

I will begin experimenting. 
Thank you for your expert advice : )