Sure, the Amp’s line in is designed to connect to any device that outputs a line level signal, such as a receiver. Most people use a tape output, but not all receivers provide such. 

The other option you may want to consider is a simple A/B switch for the RCA inputs from the Turntable’s pre-amped output and the CD Player’s output, but that’s a choice, not a requirement. 

For CDs via a player the line-level switch is the best solution.

However for listening to the music on the CDs you might find using the Sonos music library option a lot more flexible and less aggravation. I ripped my collection to a losless format years ago, got rid of the player and haven't had the CDs out from under the bed since.

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I also love the Atmos sound of a lot of music via Apple Music. But watching my daughter discover my music through my vinyl and CDs is priceless. She started using my old net md as a trend. The only thing I want is to leave things connected so he doesn't have to suffer from plugging and unplugging things. Until now, connecting the turntable to the AMP and the CD player to the Era 300s was the best option. But she is in a phase of not wanting to use apps or a phone to listen to music. 

I have a question or two:

How are you using a Sonos Amp In conjunction with Era 300’s and a Sub? You don’t need the Amp for the turntable if it has a built-in pre-amp as you can use the Sonos proprietary line-in to connect to the Era 300’s. If the turntable doesn’t have a pre-amp one can be purchased to sit between the Turntable and Era 30O’s.

You said she is in a phase of not wanting to use apps or phone to listen to music. There is no way to avoid using the Sonos app/phone/tablet to listen to any source via Sonos.