我也面临着同样的问题，我已经用以下方法解决了，我希望它也能为您工作：

Notification = new FirebaseAdmin.Messaging.Notification()
            {
                Title = "Test Message",
                Body = "You have a new Test message!"
            },
            Token = "your-device-token",
            //sound configurations for android
            Android = new AndroidConfig()
            {
                Notification = new AndroidNotification()
                {
                    Sound = "default", 
                    DefaultVibration = true,
                    Priority = Priority.High, 
                    Visibility = Visibility.Public, 
                    NotificationPriority=AndroidNotificationPriority.High, 
                   Category = "alarm", 
                   Color = "#FF0000", 
                   CriticalSound = new CriticalSound()
                  {
                    Name = "my_critical_sound", // Set the name of the critical sound
                    Volume = 1.0, // Set the volume of the critical sound (0.0 - 1.0)
                  },
                },
            },
            //sound configuration for ios
            Apns = new ApnsConfig()
            {
                Aps = new Aps()
                {
                    Sound = "default",
                },
            },

注意：我使用的是.NET 6，要实现上述方法，需要一个nuget包FirebaseAdmin，其中包含必需的类。更多细节请参考firebase的官方文档。
Link to Official Firebase Documentation
我希望这将有助于解决这个问题。