how do we differentiate between plural and singular while hearing
In learning french
The pronounciation of the noun itself almost never changes in the plural, however the pronounciation of its article does (which is why it is very important to pay attention to articles in French !), for instance:
La voiture = the car / Les voitures = the cars
voiture and voitures are pronounced the same, but la is /la/ and les is /le/, so the difference is hearable on the article.
Part of the problem that no one is addressing is that the French voice here in duolingo uses the same audio clips for la, le and les. But they are pronounced lah, leh, and lay, respectively. And this is the way you're supposed to be able to differentiate between singular and plural nouns. You'll just have to power through the lessons here and find other sources of French pronunciation. Maybe listen to a few lessons of "Learn French with Alexa" on YouTube and you will hear the difference.