By default Linux distribution are configured to use basic 2.0. If you have another audio system you will need to change the following lines from file /etc/pulse/daemon.conf:

enable-lfe-remixing = yes
...
default-sample-channels = 6

The value of default-sample-channels must be equal to the number of speakers: (2.1 → 3), (5.1 → 6), (7.1 → 8).

If your mother-board only have three jack connectors:

Green - L/R Line out, Blue - L/R Line in, Pink - Mic in. You can reassign each jack functionality via hdajackretask, and then set your Mic in and L/R Line in as L/R Rear and Front/L inputs for your 5.1.

To install hdajackretask type on your terminal:

sudo pacman -S alsa-tools 

That configuration have been tested on Antergos/Arch Linux distribtion.