understand, thanks for your attention and help

I'm not so sure you really do...
Otherwise you would have addressed that question to Google, Apple, Microsoft and friends.
Because it's THEIR mobile browsers to start in muted mode by default.

I understood, would there be a way to reverse it? if the default is to play the sound and if the user doesn't want it, press "mute"?

No,
at least on mobile devices Ossn needs that little bell being pressed by the member to confirm he really WANTS to hear a sound. See
https://makandracards.com/makandra/13893-autoplay-html5-audio-in-chrome-for-android-mobile-safari-in-ios
Although there might be a chance having automatic sounds even on Android mobile broswers in the meantime ... see
https://developer.mozilla.org/en-US/docs/Games/Techniques/Audio_for_Web_Games
and
https://developer.mozilla.org/en-US/docs/Web/Progressive_web_apps/Add_to_home_screen
it would be a real overhead, and - as said - still won't work on iPhones.

Thus, instead of confusing your members with having a bell on one device and having no bell on the other, we will stay with the current implementation (having bells on all devices) as long as there's no device independant solution in sight.