Just replace the microphone, and back with news. Regardly ,for me, it is a hardware problem , since i replace it ,and now is working just fine.Is working even better than another 1520 i've tested :). I mention that i was having the same problem, people hear me like i was under wather , only when I set it to handsfree people can hear me clearly. Changing the microphone was a little bit hard, but i've done it.Since it is not so expensive , i advice you to buy one , and test it,even if you won't fix it on the board , just plug the band on the mainboard.
I've heard that 1020 have the same problem.