The microphone on my MacBook Unibody (2009) stop working after I took it to fix a motherboard issue (which turned out to be memory module incompatible issue). Apple's HK service provider - C.A.S.E. - is doing a very very poor job. It is just as worst as with Acer in HK.
After I got my MacBook back, the microphone doesn't response at all. I tried everything, except taking the MacBook apart, and couldn't get it work. Using iPhone's handfree, the computer can get audio in. So, there is no problem with the motherboard or the OS whatsoever. I'm sure it's a problem in the microphone connection. So, I took everything out by following this guide. And I fixed the problem - unbelievable! (This is just crazy).