"Ils mangent la nourriture."
Translation:They are eating the food.
There shouldn't be any difference between the pronunciation of mange and mangent, subtle or otherwise. There are some verbs where the third person plural form sounds different, but not in most cases. A friend explained to me that the spellings reflect the fact that they used to be pronounced differently. The spoken language moved on but the spellings remained the same. What is weird about it is that when the spellings were standardized they already pronounced mange and mangent the same way, but the French have had a history of being conservative with their language! (By the way, an example of a verb which is pronounced differently in the third person plural is venir: il vient (he comes) and il viennent (they are coming) are pronounced differently.)