Two hidden menu reachable. This two is named "OEM Settings". One is for mic adjustment, and one for other things.
Just go to "Setting" -> "About device" . In there tap some times the "Model Number", I think, or an other. I can't remember, what exatly. This pickup the "OEM Settings" where You can input a password.
The two password is 1238 for MIC settings, and 1617 for other hidden settings.
No other hidden setting menu, just this two.
(Sorry for My English)
Thank you so much for that. Google assistant works so much better now.