I have had this intermittently with my Verizon G2, and my wife has had it on pretty much every call recently on her G2. For us, it only affected outgoing calls, not incoming.
I just found another post here today from other people with similar problems, and someone suggested disabling the option where the "OK Google" voice command is detected from any screen. After disabling this tonight, my wife can now be heard when she makes calls. Hopefully the fix lasts! (thanks to Irene E for the tip!)
I can't post the link to the other thread, but you can search for it under the "Verizon LG G2" section, with post titled "People I call can't hear me"
Other people have also reported success by doing a factory reset, but this may be due to Google Voice detection being turned off by default.