Find a physical switch, and attach the switch to the input on the PLAY:5. 

There are plenty of manual pushbutton devices (starting as low as $10, e.g. at amazon.com item# B00KXVBB8Q), but if you can hide it, a sound-activated auto-switch is fun.  (e.g. by MCM, amazon item# “B00L9YVL90”).   But auto costs $70, which you might instead spend towards a second Sonos speaker with a second line-in!  Just put the turntable in a different room from the CD.  Or, in the same room, a stereo pair!  The question about whether & how line-in works with a stereo pair in the same room has been asked & answered, https://en.community.sonos.com/what-to-get-228989/pairing-two-play-5-with-a-shared-line-in-source-6794046

Possibly that’s more than you wanted to spend “just to play a record occasionally”, which is the typical case; it’s tough to beat $10...

Thanks for your reply!  Yes, you confirmed what I was thinking.  I will just buy a cheap switch.

Thanks again!