Your best bet would be to split the audio out before the signal reaches the TV, by using a simple HDMI switch with an optical output, or the upcoming HDFury device, which will provide eARC. There’s a ton of threads about this solution in the Home Theater area of the forum.

Thank you Airgetlam for your answer and advices. 
But if I have simple HDMI ports on my TV, I’ve no optical port. Then how connect the optical port to ARC ? Or does exist a female optical- HDMI connection ?

Thank you in advance for your help and explanations,

BriNo

Are you sure your TV lacks optical output? What is the model #?

It is illegal to sell a TV in the USA without optical out, but you are in France so I guess you are still using SCART connections?

The HDMI switch would have the optical out port, hence the phrase ‘split the audio out before the signal reaches the TV’ that I used. 

J-Tech Digital Premium Quality 1080P HDMI To HDMI + Audio (SPDIF + RCA Stereo) Audio Extractor Converter (JTDAT5CH)
by J-Tech Digital, Inc
Learn more: https://www.amazon.com/dp/B00BIQER0E/ref=cm_sw_em_r_mt_dp_DvepFb5ZRQ0HP

Thank You very much for this link. It could help I’m sure. For reasonable price. 
I will check  Because I need to understand clearly how it works. 

Thank you for your explanation. But I still don’t understand. 
Forgive me and forget. 
I will buy a new TV : this one is to old and small. :)

Good decision!  

( ^ ^ )

Just in case you decide for any reason not to get a new TV, I thought I would explain the Extractor.

I’m assuming that your Source has an HDMI Output.   You would run an HDMI cable from the Source Output to the HDMI Input of the J-Tech.   Then you would run a HDMI cable from the J-Tech HDMI Output to the HDMI Input of the TV.   In addition, you would need a Optical cable.  The Optical cable would run from the JTech Optical Output to the Sonos Optical to HMDI adapter.  Then plug the HDMI end of the Adapter into the Sonos speaker.

All the best, 

Thank You Very Much MSterling for these very clear explanations. 
With a minor investment, I should be able to connect my actual TV with the ARC. 

Does Dolby Sound from ARC work ?

Many thanks for your help,

BriNo

@BriNo Dolby Digital is supported.  But DD+ is not.

Here’s more information that I find quite useful.

https://www.the-home-cinema-guide.com/optical-digital-audio.html