Issue
According to the documentation Set up push notifications for the mobile app
It is possible to change the sound when a Push Notification is received by the Mobile App.
6. (Optional) Change the sound for your notification by replacing the "sound" : "default" text in the Push Message Generation field with an option listed in this table. For example, use the sound for unresolved issues by entering "sound" : "NotificationAlert-4" in your JSON payload.
After setting one of the values in the "Notification sounds" chart the notification always uses the default sound.
Release
Appies to any release.
Cause
The documentation is incorrect.
Resolution
The documentation has been flagged for being updated.
Until that time you need to append .caf to the end of the value, for example:
"sound" : "NotificationAlert-4.caf"