What plugin are you using for push notifications? If you are using phonegap-plugin-push, double check their docs to make sure you have set up everything properly.
I have tried multiple things denoted in their documentation with no luck:
- Root directory of app (tried both app/ and root)
- Library/Sounds folder in app (tried app/Library/Sounds)
Did you ensure the sound file is in the correct format? How did you add the file to the project? Xcode is picky about having files added to the project, just because you copy it to the right directory it doesn't mean it'll get included in your app. It would be useful to see the logs Xcode prints when you receive the incoming push message.
I use PhoneGap Build to do my projects, and don't use XCode. As for the correct format, I have tried both .wav, and .caf, both supported file types.