What Mic Is On The Power Flex?

That's the "noise" mic. Meaning this mic will determine the level of noise next to the phone when you are placing a call, and according to the level of noise, your voice is lowered or increased.

Put a piece of tape over that mic (the whole is apparent between the back cam and the flash), and place a few calls. Soon, you should notice that the people on the other side of the line will barely hear you.

Usually, this mic is rarely used by VOIP Apps such as Viber.

So if someone cannot hear you when you place a normal call, or the sound of your voice comes and goes, but hears you fine when using Viber, usually this mic needs to be replaced.