Comment 9 for bug 1359803

I found a solution:

1. Install "alsa-tools-gui" package.
2. Run hdajackretask.
3. Check ☑ Show unconnected pins.
4. At pin 0x18, check ☑ Override and select Microphone from the list.
5. Click "Apply now" to test if it works.
6. Click "Install boot override" to make it definitive.

In my case, it was pin 0x18. I don't know if the pin is different for other laptops.

See also a screencast of this solution: https://youtu.be/yx33W-c4Cmg