Notifications fail to display in Android Oreo (API 26)

Per the comments on this Google+ post:

those [warnings] are currently expected when using NotificationCompat on Android O devices (NotificationCompat always calls setSound() even if you never pass in custom sound).

until the Support Library changes their code to use the AudioAttributes version of setSound, you'll always get that warning.

Therefore there's nothing that you can do about this warning. As per the notification channels guide, Android O deprecates setting a sound on an individual notification at all, instead having you set the sound on a notification channel used by all notifications of a particular type.

Starting with Android O, you are required to configure a NotificationChannel, and reference that channel when you attempt to display a notification.

private static final int NOTIFICATION_ID = 1;
private static final String NOTIFICATION_CHANNEL_ID = "my_notification_channel";

...

NotificationManager notificationManager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
  NotificationChannel notificationChannel = new NotificationChannel(NOTIFICATION_CHANNEL_ID, "My Notifications", NotificationManager.IMPORTANCE_DEFAULT);

  // Configure the notification channel.
  notificationChannel.setDescription("Channel description");
  notificationChannel.enableLights(true);
  notificationChannel.setLightColor(Color.RED);
  notificationChannel.setVibrationPattern(new long[]{0, 1000, 500, 1000});
  notificationChannel.enableVibration(true);
  notificationManager.createNotificationChannel(notificationChannel);
}

NotificationCompat.Builder builder = new NotificationCompat.Builder(this, NOTIFICATION_CHANNEL_ID)
  .setVibrate(new long[]{0, 100, 100, 100, 100, 100})
  .setSound(RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION))
  .setSmallIcon(R.mipmap.ic_launcher)
  .setContentTitle("Content Title")
  .setContentText("Content Text");

  notificationManager.notify(NOTIFICATION_ID, builder.build());

A couple of important notes:

1. Settings such as vibration pattern specified in the NotificationChannel override those specified in the actual Notification. I know, its counter-intuitive. You should either move settings that will change into the Notification, or use a different NotificationChannel for each configuration.
2. You cannot modify most of the NotificationChannel settings after you've passed it to createNotificationChannel(). You can't even call deleteNotificationChannel() and then try to re-add it. Using the ID of a deleted NotificationChannel will resurrect it, and it will be just as immutable as when it was first created. It will continue to use the old settings until the app is uninstalled. So you had better be sure about your channel settings, and reinstall the app if you are playing around with those settings in order for them to take effect.