Mine went to garbage at some point around or with the 6.0.1 update. I've tested it all kinds of ways and all different kinds of settings, but I think it's hardware, not software. There are 3 mics on this thing. The rear one everyone is talking about, 1 in the top speaker and 1 in the bottom speaker (where you talk off speakerphone). A few days ago I noticed that when I turned my speaker volume down (the speaker in my ear) the echoing goes away. It's almost like the microphone in the top speaker should be off when the phone is not on speakerphone, but it stays on and it can't figure out which microphone to listen to, and the echo the other person hears is because the mic in my ear is picking up their words.
Turn the speaker phone on, the sound comes from the bottom speaker. There are 3 different types of mics in this thing, and I think one of them is faulty in my case. Especially considering that covering the rear mic up does nothing to solve the problem. I hope my replacement fixes it. This is definitely not the norm, and, like I said, this one didn't do it out of the box. I've also tried cleaning the speakers and the rear mic with rubbing alcohol and compressed air.