ios10 custom push notification sound not playing

Try installing your app using Test Flight and after implementing initial process to update device token just restart your device.

You'll get what you have been looking for.

Credit goes to - binncheol

This problem seems to only affect apps installed via Xcode. The issue persists even if you later update the app from a different source.

The solution is to completely remove the app, and reinstall it from TestFlight or the App Store.

In effect, this bug should not affect your end users.

Edit 2016-10-15:

Unfortunately it seems that I was mistaken. This appears to be a bug on iOS 10 that can, in fact, affect App Store apps also. If it is of any consolation, apps such as Facebook and WhatsApp are affected too. The solution is apparently to force restart your phone, but unfortunately there doesn't seem to be much we can do about it as developers.

Apple Fixed Issue:

Based on this comment, Apple has seemed to fix the issue in iOS 10.2