Does this turntable have a built in pre-amp, that you have switched from ‘phono’ to ‘line’? If not, you will need to get an external pre-amp (inexpensive device) to boost the signal so that the Sonos can read it. 
 

Cette platine a-t-elle un préampli intégré, que vous avez basculé de « phono » à « line » ? Si ce n'est pas le cas, vous devrez obtenir un préampli externe (appareil peu coûteux) pour booster le signal afin que le Sonos puisse le lire.

As an example, https://www.amazon.com/preamps/s?k=preamps. I don’t use one myself, but they are easy to get pretty much anywhere, at any stereo shop, and don’t have to be purchased from Amazon. 
 

À titre d'exemple, https://www.amazon.com/preamps/s ? K=préamplis. Je n'en utilise pas moi-même, mais ils sont faciles à obtenir à peu près n'importe où, dans n'importe quel magasin stéréo, et n'ont pas besoin d'être achetés sur Amazon.

Hello Airgetlam, thank you for your quick response.

Yes, the Sony turntable is well equipped with an integrated preamp.

For information, it works very well on a speaker other than Sonos…

By any chance, do you have these Era 100s set up as ‘surround’ speakers?
 

Par hasard, avez-vous ces Era 100s configurés comme haut-parleurs « surround » ?

No I don't think so, I only have one Era 100

Makes sense, although why it isn’t recognizing the line in is a mystery at this point. I think I would call 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.

C'est logique, bien que la raison pour laquelle il ne reconnaît pas la ligne soit un mystère à ce stade. Je pense que j'appellerais directement le support Sonos pour en discuter.

Lorsque vous parlez directement aux gens du téléphone, ils disposent d'outils qui leur permettront de vous donner des conseils spécifiques à votre réseau et à votre système Sonos.

Did you buy the Sonos USB adapter?

Many folks seeing issues like yours have gotten a USB output device, not the needed input one.

Hi ​@MehdiYa 

Welcome to the Sonos Community!

Sorry to hear of this issue you are having with Line-In not appearing in the Sonos app. 

Before you call in, do you have Pi-Hole on your network? If so, please see my article about setting it up to work with Sonos, as I had the same issue and adding *.app-management.com to the whitelist solved it.

I hope this helps.

Edit: Though, I now think ​@Stanley_4 might have thought of the right answer there, after reading your opening post again.

Thank you for your response.

It's an Apple adapter

This one?

https://www.amazon.co.uk/Apple-USB-C-3-5mm-Headphone-Adapter/dp/B0D7MN4W7H?th=1

If so, this is indeed going the “wrong way”.  You need the Sonos adapter.

Yes, it’s this one. So based on past experiences with other users who encountered the same problem, can you assure me that by purchasing the Sonos adapter it will work?

I don’t work for Sonos, so I can’t give you any guarantee.  Indeed, how could Sonos guarantee this when it depends on things outside Sonos’ control?  I can only say that the one you purchased was certain not to work, because it is designed for a completely different purpose.

The Sonos adapter is designed for your intended purpose.  I think it will work.

People would be complaining in here incessantly if the Sonos brand adapter didn’t work on the Sonos speakers it was made for.

Hi ​@MehdiYa 

To clarify things a little for you, the Apple adaptor that you have will be a DAC (Digital to Analogue Converter) for taking a digital audio stream and turning it into a signal that headphones can use.

The Sonos adaptor is a ADC (Analogue to Digital Converter) which does the exact opposite, so, it will work as long as everything else is working too.

We’re not forcing anyone to buy our adaptor either, before the conspiracy theorists join the chat - if you can find a non-Sonos ADC that uses the same microchip as ours, it will be supported by the Linux kernel running on our speakers (I have no idea what specific chip that would be, however).

I hope this helps.

You are a wonderful community.

Thank you very much for all these answers and your kind advice. I have just ordered the adapter which I should receive in 4 days (minimum because the snow has fallen on French territory in recent days...)

Thanks again!

I am sure we would all like you to post back on how it goes, hopefully with news of success!