IOS 8 Xcode customer push notifications

At a bit of a loss now.

I want to be able to "change/update" a push notifications audio file on a iPhone that is retrieved from a server so I don't have to bundle the new audio file that has the new sound and redeploy the application via iTunes as a update.

Example. I have an audio file that plays a sound of a bell which is called by the push notification, this audio file is called audio1.m4a, all good and works.

In the iPhone I want to hit a refresh button to then connect to a server, pull down a new .m4a file that may overwrite the existing audio file with the new contents so when the next push notification is recieved the "same" file name is executed but is it plays the new sound and "not" the sound (bell).

Happy to even get this working buy replacing the file even or using a new file.

I can pull the file down from the server and store it locally in the "documents directory" but cannot seem get the push to play the new file or even get push notifications to play sounds in the "doucments" directory in the phone.

Almost at the end of my stick after hours and hours of research.... :( please help.