Is it just me who cant hear the difference between mann and mannen
A lot of the e-s that gives nouns plural endings and definite singular ending (like mannen), the e gets swallowed up and turns into a silent syllable. It comes out like mann'n. Ball would become ball'n. Hopefully you will hear that they are indeed two syllable words. Mann and ball are just one syllable.
It's hard for me too... but I figure they'd be less likely to say "Man eats..." (though Duolingo does like to get philosophical) so if it's not "En mann" it's probably hidden back there somewhere.