MacBook Air microphone volume extremely low
After closely looking on microphone input on the left side of my MacBook Air, I have found that the little holes are blocked with dust or something like that.

So I took the needle and removed the dust from microphone holes, and it's now working well!

Note: I know this would seem like obvious solution, but the holes are so little you cannot see anything in them unless you look really closely (or, more probably, use magnifier).