not sure if anyone else is still having this problem, i certainly was.. i tried doing the business card trick and it didn't really do anything.
however, during my in-call testing, the echo that the other person was hearing went away while holding my finger over the noise cancellation mic, and immediately came back when i took my finger off again. so i ended up putting a piece of tape over the hole and it has seemed to solve the problem for now. i used a small piece of thin metallic silver tape from my job and it matches great with the silver bezel.
aokp jb 4.1.2 from 12/22/2012
noise cancellation checked in phone settings
i will report back if anything changes.
pic for reference-