This happened to me too. You have to go into either your console or family settings, I can't remember which and I don't have my 360 in front of me to look at. You'll have to explore in those two categories and find something labeled voice options or mic options. Once you've found that you have to change it to from friends (the default setting if you put your age as under 18 on your 360) to everyone. Also to be able to tell if people have mics in Halo 3, just look beside their names in the pre game lobby. Lastly, in social games you have to press the directional pad up before you can talk to your teammates.