Custom Sound With FCM on Android

[momoDragon]

I have added a file sound to this directory

and my payload for cloud functions is

     let payload =

{'notification': 
 {
                title: `title`,
                body: `body`,
                icon: 'default',
                sound: 'alert',
                priority: 'high'
            }
};

i also tried sound: 'alert.mp3',

However, the default notification sound is the one that is playing.
Any idea how i can get it to play a custom sound when notification is sent? am i doing something wrong?
i followed this:
https://stackoverflow.com/questions/54002617/custom-sound-push-notification-does-not-work-flutter

version:
firebase_messaging: ^8.0.0-dev.11