Couple of things to start testing. 

First, set the TV’s output to 50 % volume, to ensure there’s enough signal for the Sonos to recognize. The Sonos line in is designed for a line level input, not a variable (volume controlled) one such as a headphone port. 

Next, check the line in’s sensitivity. A good reference for this is here. 

There is no Sonos 5 of any type. There is a Sonos PLAY:5 Gen 1 and a Sonos PLAY:5 Gen 2, and a Sonos Five.

Make sure you’re using a line in port, and not a headphone port, and that the cable is fully seated.

That ‘equalizer’ moving that you’re seeing doesn’t mean much, it isn’t actually doing any analyzing unfortunately. I’d agree it’s somewhat confusing. 

Also make sure 100% that you have plugged the 3.5mm lead fully into the back of the 5 / Five. 

As a reality check, temporarily attach an actual set of headphones to the TV. This will prove that there is headphone output from the TV.

And, of course, make sure you’ve selected “line in” as your source. You can set line-in auto-play, for future convenience. 

Feedbacks, it’s a Play 5: Gen 2. I didn’t have headphones available so I plugged to a JBL Flip 4 and sounds come out fine. I also made sure that volume was above 50 min from tv.

Conclusion: The Play 5 stays deaf.

Then I’d heartily recommend a system diagnostic while the PLAY:5 is showing in your controller, and preferably within 10 minutes of trying to use that line in, and calling Sonos with that diagnostic number. 

Support feedback is Play 5 is not conceived to work with a headphone output.  We can close topic.

While there might be some issues with the TV integrating headphone output and HDMI-ARC signal flow, unless you have a very weird TV, there is no problem with signal levels between the headphone jack and the FIVE Line-In.

Support feedback dont make sense. They say that line-in of Play 5 is conceived for hifi elements like a cd player. I asked, would the headphone output of a cd player work on the Play 5. Answer is yes.

TV is an LG oled 55e9pla whose headphone line works fine on a JBL flip 4.

What they’re saying is that a headphone output is a variable output, Sonos ‘prefers’ (is designed for) a line level input. If you follow the instructions in the first reply in this thread, along with the several posts below it, you should be fine. 

Next check… plug something else into the 5 such as a phone or tablet which has an output (probably headphones), CD player, DVD (audio out), radio, tuner… anything with an output and see if you can get it playing. Don’t rule out a faulty line in.

Oh, and just as a precaution, if that Sonos PLAY:5 is set up as a surround speaker in your system, the line in is disabled. I wouldn’t think this would be the case in your setup, but still.