Both mics not working for calls, fine for everything else pretty much?

This is really perplexing to me, and the first time I have experienced something like this that I can't figure out.

I have a iphone 4 that I recently bought to replace the screen and sell, but I have run into an issue with the mics.

Before changing the screen, the guy I bought it from said he had an issue with both mics not working. Well, after changing the screen and getting it all back together, I "reset and erased all data", and everything seemed to work fine....then after a day or two, they stopped working again.

Long story short, I have tried fresh installs of ios7.0.3, "erasing and resetting" many times, and even put headphones in and out of jack, AND changed the audio jack/volume flex TWICE, but to no avail...no matter what, I get the same thing:

Bottom mic:

1. Voice Memo always works

2. Voice Control does pick up my voice, but seems to hear the wrong commands

3. Mic does NOT work during regular calls

4. Mic DOES work for calls with Viber

Top Mic:

1. Facetime works every time

2. Video Record works every time

3 Mic does NOT work during regular speakerphone calls

4. Mic DOES work during VIBER speakerphone calls, EVERY TIME.

a couple while testing speakerphone and regular calls, I blew and sucked air in and out of the top mic while in a speakerphone call, and it would "break through" on the other end for a second or two, occasionally....so now I'm thinking maybe both mics are just messed up, and I should replace them?

This is frustrating the !@#$ out of me, and I really hope someone can shed some light onto this deal.....Thanks in advance for any help with this one!