I have a new Asus X540S and the mic is not working.
It's enable in sound/input, is not mute. I try this:
sudo apt-get install alsa-tools-gui hdajackretask
Then: Show unconnected pins, override pin 0x12 to internal mic. Install boot override.
It worked in Ubuntu 14.04
Edit this configuration file:
sudo nano /etc/modprobe.d/alsa-base.conf
And add this line:
options snd-hda-intel index=0 model=laptop-dmic