streaming from a line in connection

You do need the Sonos adaptor, of course. And if, as ​@Stanley_4 has said, the Era 300’s are in use as surrounds, use the adaptor in your 100 instead. And yes, you can stream from the 100 to the 300’s. 

It’s called grouping.

Thanks for the advice. Now to choose a cd player!

If you don't have a CD player consider instead ripping your CDs and using the Sonos Music Library. 

Rip to a lossless format and you'll never need to do that again.

My CD player and CD changer are both gone now and my CD collection is in storage. 

Thanks, Stanley. I'm looking into that option, but finding it quite difficult, so this way is a short term solution.

When I originally ripped my library, virtually every computer had a CD reader. I installed a ripping application on several computers and piled CD’s in front of each computer. One really ancient machine was better with damaged CD’s. I had the ripping programs open the disc tray when finished. Between tray openings I could clean CD’s and edit metadata. In this setup I could rip 20-30 CD’s per hour.

I no longer need mass ripping sessions, and computers with CD readers are not so common. When I have a few CD’s to rip, I’ll just stack them on my desk and occasionally feed the tray. Ripping programs are much better now and I don’t usually need to fuss with metadata and artwork. Ripping hardly impacts my regular workflow. Of course my ripping progress is not 20-30 per hour.

The ripping or the Music Library, hopefully not both?

When you start ripping, use Exact Audio Copy and rip to FLAC.

I have used Exact Audio and have linked Sonos to my music folder (temporarily on my pc), but I see no files in it on Somos, although they are OK on the pc. That's where I am struggling at present. 

Thanks. I’ve sorted it now. I am using iBroadcast as a cloud storage and keeping a colpy on an external disc locally. 

Did you follow https://support.sonos.com/en-us/article/add-your-music-library-to-sonos

Yes I did. I could get to my pc drive ok, but not an attached external drive (not a NAS). But I am OK with iBroadcast now. Thanks.