Currently, sound can only be `'default' | null`. But as far as I researched, there are ways to use custom sound files in build as the notification sound.
Suggestion: Sound files (preprocessed for the format requirements) can be all put in a folder, then specify the folder path in `app.json`. Expo should be responsible for bundling them into the app. And once the files are bundled, we can use `sound: 'default' | null | 'filename.xxx'` to customize the sound when sending notification.
Use case: This can be really useful in some scenarios such as playing a long alert for a merchant, to notify him that there is an new order.
Related links:
iOS:
- https://developer.apple.com/library/content/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/SupportingNotificationsinYourApp.html#//apple_ref/doc/uid/TP40008194-CH4-SW10
- https://stackoverflow.com/questions/37536591/change-push-notification-sound
- https://medium.com/@prakashmaharjan20/play-custom-push-notification-sound-ios-a767748a46
Android:
- https://stackoverflow.com/questions/30072269/how-to-make-push-notification-with-specific-sound
- https://stackoverflow.com/questions/38882100/android-gcm-notification-using-sound
