Swap the turntable’s cables left-to-right on the Port’s inputs. Does the problem switch sides on the Play:5 pair? 

Yes it did. 

Then the problem is outside the Port. Either in the turntable itself or the connecting cables. 

Basic fault isolation.

Problem is I've been line-in direct to the Play 5s the last week with the same turntable and cables and it worked fine. The only new variable is the Port. I'll try the Sonos cables and see if that does anything. Thanks 

The Play:5 takes a 3.5mm input. The Port takes 2*RCA. Different cables.

I was using an adapter on the Play 5

I’d still suggest you try alternative cables. 

I tried the ones that came with the Port. Same deal. 

Nevertheless, if the problem switches sides when the cables are swapped L<->R something upstream of the Port must be causing it. If replacing the cables has no effect I’d be suspecting the turntable. 

I figured it out. It was how I was using the app. I was grouping the Port and the Play 5s when I should have been using the Port as a source under line-in. They really leave that open to interpretation. Oh well, I'm good at other things

Thanks for the assist 

That’s strange. Having the Port play its own Line-In, grouped with a Play:5 pair, should have played both channels as normal. Only if the Play:5s were unpaired could one work and the other not.

You need a pre-amp between the TT and the Port. It can be outboard or built into the TT. After correct setup you would designate the stereo paired Play 5’s (or any Sonos...either single or stereo paired) as the auto play room.

Built in preamp. It was using the Port as the Room instead of the Play 5s that was the problem. Works great now