When the Era 300 is used as a surround speaker, the front-facing driver is disabled. This is by design. Even if the front-facing driver was enabled, dialog still would not play from the Era 300. Dialog is usually only played through the sound bar in a home theater setup. This is also by design as the person who is speaking on the screen is usually speaking in front of you, not behind you.

I have an oled 65 inches in a room of 20 square meters and I assure you that the performance of the system during the dialogues (95% of the time in every film or other tv program) is really disappointing ( like that of the television without home theater …) 

What model TV do you have? Have you performed Trueplay tuning?

SAMSUNG QE65S95CATXXU 65" Smart 4K Ultra HD HDR OLED  ,

I did Trueplay…..:(

Set the TV to these settings:

HDMI-eARC Mode: Auto (Off if only using the TV’s native apps)

Digital Output Audio Format: Pass-Through

As stated, if you are expecting main dialog through the Era 300’s, or any surround speakers, you are always going to be disappointed.  That’s not the way surrounds work.  Surrounds are for ambient noises (traffic, rain, forest sounds), and directional cues (bullets whizzing by, traffic or planes passing from front to back, off-screen voices).  That said, the settings for your TV have to be correct to take full advantage of surround enabled soundtracks.  After checking the suggested settings given by @GuitarSuperstar, check the Now Playing screen for your TV room.  The icon in the lower left should say “Dolby Digital 5.1”, Dolby Atmos”, or “DTS” to be getting true surround sound.

There are some posts here asking for Sonos to add a independent volume control for the center channel on an Arc, haven’t seen one for the Beam.

Would that help you offset poorly mastered sound on some media where the dialog is too weak?

the clear feeling is that I have bought a Ferrari but that it can only go like a city car…

is possible add two era100 as a stereo pair to have more lively dialogues? Can they work together well with era 300 beam and sub mini?

Perhaps watch this video below, which tries to explain what the Era’s can do when they are setup as Home Theatre surrounds… I found this interesting:

Do any of you have the same configuration as me and are happy with it? help me set up mine?

You already maxed out the number of speakers you can have in a home theater.  Surround speakers of any kind do not produce any dialog unless it is coming from off screen.  It's not the speakers, it's the very nature of surround sound.  As stated, if you expect any surround speaker to improve dialog, you are going to be very disappointed.  You need to change your expectation.

Also, the clear feeling is you are expecting something which isn't possible.  It has nothing to do with Ferrari surround speakers or city car surround speakers, neither one can do what you expect.  Also, you just may have the settings wrong on your TV.  Did you check those settings as advised above?

To stay within your anology: you’ve bought the base model Ferrari and want to use as a submarine. A surround system is what is says on the tin: to be used for surround. Rear speakers will do surround sound: helicopters flying away, applauding audiences, birds in a park, exploding bombs, that kind of thing. Expecting them to play dialogue (that is placed on the front speaker by the sound engineer), however fitting to your situation, will lead to disappointment, as you experienced. This disappointment is not caused by Sonos, but by your expectations.

I think this could be helpful

Perhaps it would be better for Sonos to make two physical speakers, each with different number of drivers, and use one type for surrounds, and the other type for standard speaker use. 

The number of people who seem to want a muddier soundstage is interesting to me. 

It’s a phenomenon I’ve seen in A/V audio for years, “I paid for all those drivers, I want them all in use at all times, no matter how bad it sounds!”.  Right in line with the “I should be able to use as many speakers as I want, no matter how bad it sounds!” meme.