Hi, Sorry for the long post, but it's worth it to a lot of users! This has been of a disturbance to me for quite some time.. Possibly the thing that most disturbs me about the iPhone 3GS is the that while talking in GSM on speaker mode (hands free), the other side usually complains about not hearing well, unless I hold the phone so the speaker is directly in front of my mouth. Then again, holding it ruins the whole point of hands free. My guess is that the noise cancellation, used when switching to speaker, is what's causing this. It's just too harsh. I did a little test, and the problem is not that bad when using VOIP apps such as viber. Maybe these apps don't employ the same noise cancellation schemes as the native phone app. Also everything sounds fine of course when not in speaker mode. All this means to me it's not a microphone hardware problem. I tested with several 3GS's. I don't think I had this problem with the iPhone 3G. I'd rather have the other side hear some noise as long as they at least hear me. I will bless every night anyone who knows of a way to fix this. Jailbreak fixes are also blessworthy.