It said "You are hearing me" was incorrect, and that the correct one was "You all hear me".
Where does the "all" come from? What's the difference?
"You are hearing me" is natural in English in certain circumstances. It's not as frequent as the continuous forms of most verbs, but it's certainly not impossible. Some language references take the first observation in the prior sentence and blow it out of proportion to say that this sort of phrasing is impossible. As a native English speaker, I call that bunk. However, the Dutch team seems to have gone along with it.
"You are hearing me" is incorrect because to hear is non-continuous, hence you cannot use the -ing form of it.
"You hear me" would be correct as well, as the "all" would be implicit with you in plural when nothing more is specified.
For example "You three hear me" specifies three specific people, even if there might have been five people present. "You hear me" implies you talk to all, hence "You all hear me" is the implicit understanding of the sentence.
The previous phrase was "jullie horen MIJ" and this one "de jongens horen ME niet". Can somebody explain me the difference please?
The same difference between all stressed and unstressed pronouns. Mij is stressed, me is not. Either is possible.
Why isn't 'me' acceptable since how can I tell if the action is stressed or unstressed
It is accepted, unless it was a "type what you hear exercise". You have to type exactly what the audio plays.