A few months after the launch of the Galaxy S2, a small but significant number of users are reporting mic and speaker problems on their phones. Sometimes the phone doesn't turn the speaker or mic on during a call. Other times, the speaker makes a bunch of unintelligable noises. All the while, users report seeing full bars, leading them to believe it's a hardware, not carrier, problem.
Here's a typical complaint, from SamsungGalaxySForums.com:
When I am calling someone the phone sometimes does not turn on the speaker and mike. The call always goes through no trouble with that at all. The phone is ringing to whoever I am calling, but the one I am calling cannot hear me and vice versa.