One day, my MSI GT70 laptop simply would not detect external microphone (on my headset) any more. I tried to update Realtek audio drivers, looked up any online forums but nothing worked.
I noticed that if I hover my mouse over the microphone jack (the pink one in below diagram (although on mine it shows black)) the text says ‘Center & Subwoofer’ instead of microphone. So I thought there must be something wrong at the configuration, and it turned out that it is simply the Speaker Configuration (combo box / drop down list) in the below diagram.
If you choose more than 4 speakers, the microphone jack will be used for audio output for the center speaker instead of for microphone input. So the solution is as simple as changing the Speaker Configuration to quad… or less and that will free up the pink jack for microphone.