I have listened to a number of launch countdowns, and especially as they poll for readiness, a large number of people (10-12 or more) need to respond when called on, but the sound quality is excellent.
I seem to live on conference calls for work, and people usually stink at muting/unmuting and background noise is almost always an issue.
I cannot imagine that Mission Control is a quiet place. Yet the countdown net usually has excellent sound quality, everyone responds quickly and on time.
What sort of headset/technology are they using to accomplish this?