So, I often play Doubles in Reach, and when I do play, I always play with the same friend. When we're sitting in the pre-game lobby, none of the players (other than my friend) have a mic on my screen. However, my friend will be having conversations with the other people as we are sitting there, so they obviously do have mics, I just can't see/hear them. The usual culprit is a closed or moderate NAT, but I have an open NAT status. My second thought was maybe privacy or child settings (my account was, at one point in time, a child account), but looking into those I have voice chat on for all people.

What could be causing me to not be able to hear any other people in matchmaking?

Of note, on occasion I run into people who show a muted sign, but never anyone who has a mic symbol. Also, voice chat works correctly in other games (like Mass Effect 3), so I think it might be specific to Reach.