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.