Maybe an internal vocal system could be nice. In HD sometimes it’s hard to comunicate with youre mates when you have to micro youre unit a lot at the same time.
I disagree, for as far as I know, all the internal vocal systems are bad quality or just random people starts yelling. Besides that there are almost always some players that muted everyone. I would recommend a channel at discord for all AOE:DE players, there you can team-up with other players and create your own room.
I find its a good idea, but i dont know that the Engine can handle this
I want to believe that aoe community is composed by respectfull people.
From what I’ve seen of this forum there’s a substantial component of incoherent people.
Although most of that may be language barrier, by virtue of AOE 1 being a global phenomenon. 
I wouldn’t mind optional in-engine voice chat but at this point in development I think it’s unlikely and a low priority. Besides, people can set up Mumble servers on their own, or, even more easily, just use Discord.