paragraph1\phpFCM\Notification::setSound PHP Method

android/ios: can be default or a filename of a sound resource bundled in the app.

See also: https://firebase.google.com/docs/cloud-messaging/http-server-ref#notification-payload-support

public setSound ( string $sound ) : Notification
$sound	string	a sounds filename
return	Notification

    public function setSound($sound)
    {
        $this->sound = $sound;
        return $this;
    }

Notification