Microphone not working in call
My Microphone won't work during a call unless I activate loudspeaker mode.
I dropped it in wet grass recently which may be the cause
Mic works on a voice recorder app or while taking videos.
Back up your data and factory reset your phone, if that doesn't fix the issue I'd say you're looking at a faulty Mic. You'll find the part on most site like ebay and even if you ordered it directly from LG. You'll also find disassembly guides for your phone here on ifixit.
I fixed my issue by cleaning out the microphone hole with a needle. The hole was filled with grunt, once removed the microphone worked.
Be careful not to stick the needle to far in.