"Les filles mangent du riz noir."
There is no difference in pronunciation. As it is "les filles" the ending for the verb should be -ent. If it was "la fille" then the verb would end in -e. The pronunciation for je mange, tu manges, il/elle mange, and ils/elles mangent is the same. You don't pronounce the "s" on the end of manges or the "ent" on the end of mangent. Hope that helps.
Because they aren't eating "the" rice. They are just eating rice. Du = "de le" = some. If in context you were upset because the girls were eating YOUR black rice that you spent ALL DAY making, and you ran to your friend to complain, then you would say "Les filles mangent le riz noir!" - "The girls are eating the black rice!". But since it's just rice, it is "du riz". Hope that sort of helps!
Oh, okay. So, I assume that is also the case with this translation that actually doesn't make much sense because men are technically men, not boys. "The men are boys." --> "Les hommes sont des garçons." Is there a relation (meaning that if the English sentence had rather been, "The men are the boys.") there between using "des garçons" in the previous sentence and using "du riz" in the sentence concerning the black rice?