Android Notification Sound Allow, disabled by default.

Please provide the following:
SDK Version: 43.0.0
Platforms(Android/iOS/web/all): Android

Hello, we are developing an app that has push notifications.
We send the push notifications via PHP.
The notifications arrive perfectly to iOS, but when they arrive to Android, the notifications have no sound.
They show perfectly, they vibrate perfectly, but no sound.
After looking under notifications on the App Info, we saw that it comes with the “Allow sound” option disabled.
It happens with all the apps we build.

We have been looking around and found this thread ( Android App Notification Sound is disabled/off by default ) where they describe the same problema, and one of the answers given by one user is that:

WhatsApp , Telegram , Snapchat , and so on. These are all white-listed apps which means the package names are hardcoded at OS level to allow some of the permissions enabled by default. Our apps are not so. Hence, we need the user to enable those permissions manually.”

Is that true? It makes no sense for us…

We have looked so many pages, and tried so many things and none of them works, everytime we make a new build, the sound option comes disabled.

We tried as many people suggested, creating a channel, without success ( the channel gets registered, but the app remains without sound permission ).

  Notifications.setNotificationChannelAsync('default', {
    name: 'default',
    sound: true,
    importance: Notifications.AndroidImportance.MAX,     //Here tried with 'max' only also, same result
    vibrationPattern: [0, 250, 250, 250],
  });

We also though at some point, as we use phones with MIUI for testing, that this could be caused by MIUI, but tried on phones without it, with the same result.

We cannot give an app to the client and tell him to ask everyone of his users to manually enable the push notification sound.

Thanks