android_notification_sound.cs

var pending = PendingIntent.GetActivity(context, 0,
    resultIntent,
    PendingIntentFlags.CancelCurrent);

var builder = new Notification.Builder(context)
  .SetContentTitle(title)
  .SetContentText(message)
  .SetDefaults(NotificationDefaults.All)
  .SetSound(RingtoneManager.GetDefaultUri(RingtoneType.Alarm)); 

builder.SetContentIntent(pending);

var notification = builder.Build();

var manager = NotificationManager.FromContext(context);
manager.Notify(1337, notification);

MediaPlayer player;
player = MediaPlayer.Create(context, Resource.Raw.CustomToneName);
player.Start();