ASUS Zenbook UX431
Volume increments for the speakers do not work
Because of the "surround sound" audio hardware on the UX431, you have to tweak which pins are used to get the volume incrementing to work. Otherwise, the volume will do two things: lower the volume of the high frequency sounds (the two front facing speakers are the ones that emit that) and, when mute is reached, mute the four speakers.
To do so, install
From the HDAJackRetask GUI, make sure you have the correct codec selected on the top part (Realtek ALC294), then select, on the top-right corner, "Show unconnected pins". Enable the pins 0x1b and 0x1e and assign them "Internal speaker (Back)" and "Internal speaker (LFE)" respectively. Lastly, click on "Install boot override". This will add a configuration file in /etc/modprobe.d/ called hda-jack-retask.conf with the appropriate contents. Once rebooted, the computer should allow for volume control.
Headphone jack microphone is not detected
This is a known issue. You can use the internal microphone (on top of the screen). Feel free to edit this page if you have found a solution.