How can we understand what babies are crying about? Does their cry sound different when they are hungry compared to when they are tired?

Yes, you can know base on you yourself have to manage through experience. Every baby are different, you may can check though those example and slowly understand your baby cry. 1st- hungry 2nd- diapers wet 3rd- rashes (body/buttock) 4th- stomach bloated 5th- teething 6th- hot Etc... 7th- if cry non stop better go see doctor.