Gentoo, ALSA, mute speakers when headphones are used

Lenovo G400s 

Specifications:
lspci: 00:1b.0 Audio device: Intel Corporation 7 Series/C210 Series Chipset Family High Definition Audio Controller (rev 04)

cat /proc/asound/card0/code* |grep Codec :
Codec: Conexant CX20757
Codec: Intel PantherPoint HDMI

Kernel configuration: CONFIG_SND_HDA_INTEL=y and a hosts of other CONFIG_SND_HDA_xxx options.

I initially encountered a problem where the laptop speakers do not mute when I use headphones. As usual, the solution is very simple. 

Bring up the configuration for sound settings by typing alsamixer in the terminal. 

Enable the Auto-Mute Mode.