There’s nothing physical, no. Generally speaking, you’d use the volume control on the device that the Port is feeding. Is that not a possibility? All the Sonos is doing is providing a line level, not an amplified (level adjustable) output.

Normally you would use the volume control on your receiver to control the volume so the Port has no touch controls.

You can change that and use the Sonos App or one of the other Sonos compatible volume controllers, 

What is a compatible volume controller? If I do whole home play of multiple speakers I want the ability to walk in the room where the port would be feeding a pair of Meridian powered speakers and adjust the volume from the room without having to have the phone by my side. 

Searching a bit may find more but a quick look found these

https://www.ikea.com/us/en/p/symfonisk-sound-remote-gen-2-30527312/

https://www.casetawireless.com/us/en/sonos

Thank you Stanley, I will investigate further.