There are a number of things which could be stopping the sound from working.
1, If 'Do Not Disturb' is active no sound will play
2, If the App is open at the time just a message will appear on the screen with no sound
3, If the device is muted or the ring volume turned down it will stop the sound
4, Sounds need to be enabled for the notification, check in Settings -> Baby Feed Timer -> Notifications that sounds are turned on
In some rare cases there seems to be an iOS bug where the notifications section doesn't show for Baby Feed Timer and therefore notifications do not show.
To fix this issue you will need to make sure you have created an account from the 'More' tab in the 'My Account' screen so that all your data is backed up. You will need to delete Baby Feed Timer and download it again for free from the App Store. You will be able to log back into your account to restore your data, but you will need to update your settings inside the App if you changed them from the defaults. After you do this notifications will work fine.