This is likely a blockage issue caused by a combination of factors. Inside the phone case, on the other side of the mic hole, next to the audio jack, there is a little tunnel (wave guide) for the sounds to get from the hole to the microphone. The “tunnel" is removable either by unscrewing or unclipping it (I don't remember) once you've got the case opened. The "tunnel" sits on a protective “grommet" held in place by an adhesive that's not a good choice considering the range of temperatures and moisture that a phone will be exposed to.
The adhesive is much like what you'll find on cheap electrical tape, greasy crap that sticks to skin really well but nothing else.
This protective grommet may have a permeable film spanning the center opening or it may not. I wasn’t able to determine the original condition of mine or those inside the other phones that I have repaired.
Anyway the grommet (this is my terminology and not necessarily what LG calls it in their service manual) is a ring shaped sticker that straddles the microphone opening on the pcb (circuit board). The microphone is integrated, meaning that it's soldered directly into the pcb, and is not “unpluggable" which adds a level of difficulty as we cannot just order another microphone to plug in.
So far I have seen two separate issues affecting this microphone (again, this mic is the one next to the headphone jack):
Issue 1: a combination of heat (whether internally or externally generated - leaving phone in the sun, for instance), and physical force either from blowing into the mic hole, dropping the phone, or from a foreign object sticking in and physically moving the grommet, while its glue is heated and weakened, so that it blocks the hole on the pcb. One phone that I repaired had been in a purse every day and I believe that either a hair brush or makeup brush was in there and it's bristles got pushed into the mic hole as the purse got jostled. This resulted in NO SOUND whatsoever being picked up by that mic unless she shouted or blew directly into the hole next to the audio jack.
Issue 2: Same initial grommet glue problem combined with heat, force, causes the grommet to move only slightly, but now there is exposed glue around the mic opening. Eventually dust and particulate matter will make their way into the mic hole and then be stuck on the glue- not removable by blowing into the mic, but instead it becomes even worse and causes crippling distortion or sporadic issues with mic insensitivity. This is especially true if the particulate matter happens to be made of iron or steel, such as a tiny clipping from a stainless steel wire strand. In that case, glue will not even be necessary to make it stick to the mic since mics have a magnet inside them and will attract things containing iron.
In all the repairs that I have done so far I simply removed the grommet, cleaned up the remaining glue, cleared the mic opening, reassembled the phones and they worked just fine.
In hindsight, I think it might be a good idea to include a piece of thin, sound permeable material between the pcb and the wave guide to prevent more foreign objects from entering. In fact I have seen other v20s that do have such a material in place…but some do not.
I found nothing else on the internet that correctly addressed this issue so I hope this helps.