I'm a U.S. att user. I just phoned HTC at 866-449-8358 and reported the issue. They said it had not been reported before. this is BS. Hundreds are complaining online and several have said specifically that they have called HTC to report it.
---> SO I ASK: if you're a U.S. customer, please call 866-449-8358 and report it as "muffled microphone" or "muffled call quality" and describe it for them. We have to get an official answer. I hate the idea of returning the phone and getting another one with the same problem. Or getting a different phone like the Samsung.