Is it lowering its audio, or is it dropping its audio due to buffering issues and problems with its connection/signal strength? Maybe monitor its SNR levels and ensure both Era speakers are operating at/above 45dB signal strength. See this link on where to locate the SNR level in the Sonos App:

https://support.sonos.com/en-us/article/understanding-the-network-details-section-in-the-sonos-app

@Fragments Of

Could be a network connection issue but I recommend contacting Sonos support and sending a diagnosis before. So they can take a look inside. 
https://support.sonos.com/en-us/contact

@Schlumpf 

Thank you for your help.

One more thing, I have in total 4 Era 100 : 2 in the living room and 2 in the kitchen.

I have moved the 2 Era 100 of the living room in the kitchen and the problem appears sometimes like informed in my first post. The problem appears in the kitchen.

I go to checked the network.

Kind regards.

@Ken_Griffiths

Thank you for your information.

I am not in my home, I go to checked with your information.

Kind regards Ken.

@Ken_Griffiths 

Hello again Ken,

I have checked the SNR on the speakers : the left one has 57dB and the right one has 55dB.

Kind regards.

I see you mentioned they are in the Kitchen - one common device that can interfere with their signal is a microwave. Do you perhaps have one near to either speaker? I’m wondering if that might perhaps be the reason?

@Ken_Griffiths 

Indeed, I have one microwave but when the problem appears, the microwave is not in function.

Before, I had 2 One SL also in the kitchen and I these one works properly without problems.

I have also in this room, one iRobot in 2.4ghz maybe this one > ?

The problem appears when the iRobot is not in function.

Kind regards.

@Fragments Of,

In that case it maybe best to try to reproduce the issue and then immediately submit a Sonos system diagnostic report from within the Sonos App, note it’s reference and then contact/chat with Sonos Support Staff via this LINK and see what the Staff can perhaps suggest to resolve the matter.

@Ken_Griffiths 

Thank you for your support.

I have one question, I have one Sonos Boost, maybe If I use this one the problem will be resolved, what do you think ?

Kind regards.

The Boost is likely not going to be the answer here, because when wired to the router, it creates a SonosNet (wired/wireless) network for your Sonos system and the Era speakers do not use that network, as they are designed to not work with SonosNet. 

You could perhaps try a different 2.4Ghz and 5Ghz WiFi channel instead on your router, but you seem to have a decent WiFi connection already to both speakers. If you do try different channels, then for the 2.4Ghz band use only channels 1, 6 or 11, as they are non-overlapping and to help reduce interference set a channel-width of 20Mhz only (if your routers allows that). Just keep an eye on their SNR level and as mentioned earlier, try to keep it at/above 45dB.

@Fragments Of 

I also think using fix one of the 2,4 GHz channels @Ken_Griffiths listed for your Sonos system could solve the problem. As you mentioned in your entry post it seems until now you just used 5 GHz wifi with your Sonos. So 2,4 GHz would be a nice try.