HOWTO configure analog surround sound on (k)ubuntu linux for mobo with only three audio connectors
Some budget motherboards supporting 5.1 channel audio have only three audio connectors (Light blue, Lime and Pink) on the real audio panel.
Freshly installed kubuntu 14.10 (utopic unicorn) shows only stereo output (Built-in audio analog stereo) on such motherboards.
Actually such a motherboard has rear channels out shared with line-in, and mike input with Center/LFE output.
At first, you should make speaker connections as shown on photo
1. Connect the "Rear Speaker" (black) jack to Line In (Light blue) port 2. Connect the "Front Speaker" (lime) to Line Out (Lime) port 3. Connect the "Central / Subwoofer" (orange) to Microphone (Pink) port
Now you should configure audio backend software to recognize those connected devices as surround speakers rather than aydio input sources.
(k)ubuntu 14.10 uses PulseAudio for the backend sound system.
Open /etc/pulse/daemon.conf with your favorite editor
sudo vi /etc/pulse/daemon.conf
and configure default number of output channels.
... default-sample-channels = 6 ...
Save the file.
Start alsamixer and change number of channels from 2 to 6
Exit alsamixer and reboot the computer.
(See http://drona.csa.iisc.ernet.in/~uday/surround-pulse.shtml for details on configuring PulseAudio, if you have other or older linux distro).
Now you are able to select 5.1 output from system settings GUI.
After selecting sound profile switch to Device Preference tab and make sure that Built-in Audio Analog Surround 5.1 comes first in the list of preferred devices for Audio Playback node.
You may check that this preference is not overriden in child nodes (Notifications, Music etc).
If you still do not hear surround speakers during playback, also check that no channels are muted in the mixer.
That's it