There are so many ways that this answer can be answered and it is going to be interesting in how Bungie actually answers said question. I think that it would get a little chaotic if you were able to hear every person in your world t the same time. It would be madness. Maybe they could divide it up into planets and from those planets, divide it into sections. Based on what planet and section you are in, those are the people you hear. If you are in a party, you should be able to hear them no matter where they are. But there should also be some kind of feature that allows for emergency signals to go out as well. Say that you are in a heavy fire fight and it doent look like you are going to get out alive. You send a distress call out, saying that you need help, ASAP. People hear it and come rushing to help, saving your life and pushing what ever enemies you were fighting , back.