Connect the transmitter to the Port via RCA.  That should work.

Did that.  Powered up the transmitter, connected my speakers and tried to stream some music but no sound?  Any further suggestions?

I should clarify.  I have my turntable connected to the In Port and connected the Bluetooth transmitter to the Out Port.  

Set the Volume Level on the Port to Fixed.  It may be a case of dueling volumes causing the level to be low. 

How do i do that?  I assume using the app in settings?

They now call it “Line-Out Levels” (don’t recall when that changed).

Open the Sonos app for iOS or Android.

• From the  Settings tab, tap System.
• Select the room with your Port or Connect.
• Tap Line-Out Levels and choose one of the options.

Thank you!  I’ll give it a shot when i get home.

I’m not sure I’d recommend using Fixed Volume on the analog outs into Avantree Oasis Plus. There’s a chance of overmodulation/clipping. 

Since that transmitter has an optical input you could get an inexpensive coax/optical converter and simply run digital all the way, instead of DA conversion in the Port then AD conversion back again in the Oasis Plus. 

Has anyone determined if it is possible to connect wireless headphones to a Sonos system, via the Sonos Port or any other way?  I too would like to connect Bluetooth wireless headphones to my Sonos System and independently control the headphones’ volume, etc.

The only Sonos device with an analog output is the Sonos Port, you would want to connect the Bluetooth transmitter to that analog out, and send the signal to your headphones.
 

There is no other way to connect headphones directly to Sonos, although some folks have connected their headphones to the digital output (optical) on their TV sets, changed that digital output to an analog one, and then sent that to their headphones. It has the benefit of not getting involved with CEC via HDMI, and ‘blocking’ some other audio sink. This is a pretty good solution for TV watching, not so great for just music. 

Or the Port’s coax digital out. Better BT transmitters have digital input. A good one is the FiiO BTA30 Pro. 

A really old Sonos Play 5 has a headphone jack but it is S1 only. Otherwise the other suggestions.

Thank you everyone for your helpful suggestions!
 

I am going to order a Sonos Port and try to connect to the Bluetooth headphones using a Bluetooth transmitter connected to the Sonos Port.

I will give you an update after I give this a try.

Thank you again!

I now have the Sonos Port and a FiiO BTA30 Pro with the two connected with a digital coaxial cable.  I have the Port and my Arc in a Group.  The Sennhiser headphones connect via Bluetooth to the FiiO BTA30 Pro but I do not get any sound through the headphones.  Could someone please tell me the proper way to set this up or how the settings need to be configured?

PORT’s Line-In is not automatically passed through to Line-Out. You must select an input as you would for any other player.

I don’t see any options for Line In (see photo).  I am trying to use the Arc as my input source.

The line in on the image you’re showing is not connected to anything, hence you can’t direct it anywhere. 

If you’re trying to use the HDMI-ARC as a line in, you’d need to group the Arc as your main coordinator, and all grouped rooms would be playing the input coming in from the ARC signal to the Arc. 

I’m not sure what you’re really trying to do here, the Port has essentially nothing whatsoever to do with the Digital line in on an Arc. 

Arc is in surround system with sub and two Era 300.  I then grouped this surround with Sonos Port which has digital coax going to Bluetooth transmitter which I am trying to connect to headphones.  I’ trying to send Arc sound to headphones 

The digital coax connects the Bluetooth transmitter to the Port, not the Arc

Can you send TV output to one of your other Rooms?

Check the volume level on the Port.  You may have a dueling volumes problem.

In some TV’s, use of HDMI-ARC turns off the optical stream. Have you checked that?

Yes, I can send TV sound to other Sonos speakers in other rooms.

I didn’t notice, can you play other music sources through the headphones?

Yes, I can actually connect the headphones directly with Bluetooth to the TV with low quality stereo