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