Not sure exactly how you have connected the Port to the amp (please provide details if necessary).  But you should try grouping the Era 300 and the Port.

You might be able to connect the amp to the Era 300 with the Sonos adapter.  I am not sure you need the Port.  Depends what you are trying to do.

Edit: btw, welcome to the Community and to Sonos!

Are you playing the amp through any other (passive) speakers?  If the aim is just to take the audio output from the amp to the Era 300 then the Port is probably redundant.

​@John B 

I believe the Port is connected to an AV stereo amp via RCA out from the amp to which the OP has a CD player connected. The end game is to listen to CD’s over the Era 300.

I’m not sure what you mean by “amp”. Is this the SONOS product named “AMP” or a generic 3rd party ‘amplifier’?

Thanks for all these very rapid replies!

More details: 

The amp is an old Harmon karden amplifier getting input from cd player. 

Have connected output of amplifier to input of Port.

What am I missing? 

Thanks again! 

Just group the Era 300 to the Port when playing CD’s. If speakers are connected to the HK you may have an echo effect if the HK is in the same room as the Era 300.

Thanks! This must be a dumb question but how do I group?

I can see Port and speaker in sonos app...

Not that all . Start CD music on the Amp with Port. On the Era 300 press the Play/Pause icon for a few seconds until you hear audio.

You can also set the Era 300 as the “Auto Play” room within the app as an option for the Port under Line-in (within its room).

Sorry again...don't see play/pause. 

In the Den where the Port is, it says no music queue even though music is coming through hard wired speakers. 

Play/Pause is located on the speaker itself below the volume slider between the left/right arrows. They aren’t words just symbols.

Got it.

Beeped a few times but that's all. No connection. 

What sort of connection do you have from Amp to Port?

Are there any sources besides CDs that you want to play? 

Edit: I am wondering if a better set up for you would be CD into Port and Porr into amp.

 Is the amp line out a variable preamp out?  I have a Port, but nothing is connected to it.  So the Line-In settings are grayed out.  I don’t know if Line In would show on the Sonos app Home Screen when the Port is selected as the device

to play to.  If Line-In does show on the Home Screen select it or I guess Autoplay would do that for you when the port detects a signal from the amp.  All guesswork on my part.  Other Port owners can clarify.

 If you get an indication that the Port is playing something you can group the Port with the 300.  Like ​@AjTrek1 stated though there may be a delay between the wired speakers and the 300.

If the CD player is the only amplifier source, connect the CD player to PORT’s Line-In and PORT’s Line-Out to any amplifier input except Phono.

Line-In is simply a music source on the network that can be selected to play on any combination of SONOS players. On PORT you could select its Line-In to play CD through the amplifier or any music source available to the SONOS system.

​@GreyGuy . If you stick to the configuration that you ate currently trying to make work then the Port is just a (very expensive)  means of getting audio from CDs to your Era 300. If,instead, you make the Port an INPUT to the amp  you  in effect turn your amp and speakers into another Sonos "room". Anything that you can play on the Era300 could be played on your amp and speakers.  That would include CDs, if you connect the CD player to the line in on the Port. 

I just got off the phone with technical support. They eventually concluded I could not use the Port to transmit from my old amplifier to the Sonos speaker! Said that was not what the Port does.

Which seems to contradict the (limited) info on sonos. com. I'm not happy. 

Any other suggestions? 

Thanks! 

What is the Harmon Kardon model number? 

Never mind--I figured it out. 

The Port is in the Den.

So, in the app: Den → Line-in (connected) → Autoplay → Autoplay Room: checked Den. And, in Autoplay, turned on Include Grouped Rooms.

Obviously tech support didn’t know what they were doing. And it takes a good bit of digging on Sonos.com to find all these details. 

Anyway, I’m finally set up and all works well. And sounds great.

Thanks for all the suggestions along the way.