You should be able to connect the SpinBase’s line-out to the Play:5’s line-in with a 3.5mm cable. 

But why not just connect the turntable directly to the Play:5 instead of getting a SpinBase? Why do you need both?

The SpinBase has a built in phono amp. And the simplicity of the SpinBase is something is something my kids can use. 
 

But I am assuming the Play 5 would need a phono amp - is that correct? Or can the Sonos system handle that? Thanks. 

If your turntable doesn’t have a built-in preamp, then yes, you would need to add a preamp before you connect it to the Play:5.

I’m going to see if I can make the connection to the Sonos using the single 3.5 connection to the Play 5. If it works I could get a bit better sound separation and bass from the 5 but turn off the Sonos connection when others want to just use the SpinBase. I’ll report back about how it did or didn’t work. 

Please report back. I’m curious to see if the audio from the SpinBase and Play:5 will be in sync.

It shouldn’t be in sync at all, unless the Spinbase has a delay built in to the speaker, and not the line out. The line in on the Sonos will have a minimum of 75ms, depending on the settings you choose for that line in, but never less than that.