Hi everybody. Great forum.
I also got a brand new Otterbox Defender from Ebay for my N7100. Genuine case as far as I can tell...
I modded the case so that I could slip in my 6400mAh Mugen battery.
Despite the fact that my mother, every time that I was calling on her home phone, said that she was like talking to herself because of the powerful echo...I was very happy with my case... until last night... when I checked this forum....you got the picture.... and found out that Mom was RIGHT.
I don't recall anybody else telling me about this echo... but maybe they just didn't care, since these were mostly work-related contacts...
Last night I tried to do the fix that I got here on XDA, with two slices of ear bud foam...it involved a little bit of cursing here and there, but I managed to align the holes and to trim them with an electric solder for electronic parts. It is not observable and I will post in a few hours if this did fix the damn echo or not. Cheers.
UPDATE: Guys, I called many persons, including Mom and wife. ECHO IS OFF. I confirm that this fix is working. Cheers.