Comment 38 for bug 874535

>> With debugging still enabled, I plugged in cables to the motherboard rear audio ports and it showed plugged events for 'Rear Mic Jack' and 'Line Jack', but not for the any of the Front/Center/Rear/Side audio out jacks (so I assume either the hardware or the default ALSA/PulseAudio config does not do jack detection for those).

because pulseaudio only check the line out recently , some business desktop and those all-in-one-pc have internal speakers , line out and headphone jacks

http://cgit.freedesktop.org/pulseaudio/pulseaudio/log/src/modules/alsa/mixer/paths/analog-output-lineout.conf