Unless the ERA 300 is being used in a surround setup, the simplest way is to buy the following adapter:

https://www.sonos.com/en-us/shop/sonos-line-in-adapter

That will give you a line-in for your Era 300.  Then you only need a cable from your CD to the line-in (probably a stereo RCA to 3.5 mm stereo cable).

Get the Sonos line-in adapter and connect it the the USB-C port on the back of the Era 300. Then get a CD player that has RCA or AUX out and connect the CD player to the line-in adapter on the Era 300 using an RCA to 3.5mm audio cable (RCA) or a 3.5mm audio cable (AUX).

If the CD player can transmit audio via bluetooth, that is also an option.  Again though, not when the Era 300 is used for surround sound.

As long as the Era 300 is not being used as a surround speaker, you would just connect the Sonos USB C dongle to it, and connect the CD player to that. Then use the line in on the Sonos to play the CD players input anywhere in your Sonos system. 

If it were me, though, I’d rip those CDs, store them on a hard drive or NAS, and use the Sonos library system to play them, so that I could mix and match music as I wanted, without having to get up and change the CD every time. Too much physical effort, just to play music, in my opinion. 
 

Edit: Wow, I type slowly ;)