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.