Headphone jack?  In a Five?  That’s the Line-In, not a headphone jack.  There are no Sonos devices with a headphone jack (though the Port does have an RCA line-out)

The 3.5 mm port on the Five is an input, not an output.    The only Sonos device that can output audio via wire is the Sonos Port.

The original, gen 1 PLAY:5 had a headphone jack in it. 

The gen 2 PLAY:5 and the subsequent Sonos Fives do not. 

Have you called Sonos Support directly to discuss it?

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system

The Play 5 Gen 1 had a headphone jack but the Play 5 Gen 2 and the Five does not. ​@JoRoh, is it the older Play 5 Gen 1 that you are asking about?

If so, have you checked for updates lately? I believe was a S1 update last month.

EDIT: ​@Airgetlam types faster than I do!

Sorry, you are right.

It was the old Play5 that I used with my headphones.

Didn’t expect another function on that plug …

Thanks a lot.

If I remember properly, the gen 1 PLAY:5 had two ports, one for headphones, the other (which has continued on subsequent models) for a line in.