I have the same microphone problem as everyone else with the LG G2 (tmobile 4.4.2 D80120a).
I'm nearly positive its a software issue regarding noise canceling. I kind of found a work around. First i turned off noise suppression in call setting (not actually sure if this matters). Then I plugged the top microphone hole on the phone as well as plugged the hole in my phones case for said microphone. People can hear me WAY better now. I tested it with my GF using my phone. I had her hold the phone at different angles, pointing away from her mouth, etc. I told her to talk while covering and uncovering the secondary microphone hole with her finger while speaking. Her voice would indeed go in and out. So it's my conclusion that its the phones software is confusing our talking with ambient noise or something. I could of course we way off, but it worked for me