Comment 12 for bug 874880

So after reading a bunch of different alsa-related solutions I tried to set my audio codec explicitly.
For this I followed this manual: https://help.ubuntu.com/community/HdaIntelSoundHowto

Basically I added to the end of /etc/modprobe.d/alsa-base.conf this line:
#========
options snd-hda-intel model=targa-2ch-dig
#========

At the moment it seems like my problem was solved. Without plugged-in jack sound comes from built-in speakers, while with plugged jack sound comes to external amplifier.

Here's a list with most of available model names:
=======
http://www.mjmwired.net/kernel/Documentation/sound/alsa/HD-Audio-Models.txt
=======

I picked mine for "Targa/MSI with 2-channel" based on my netbook name.

So please try to find a proper model for your audio codec and hopefully your problem will be solved.

But anyway I'm still disappointed why I didn't have this issue before 11.10 (10.04 - 11.04 worked perfectly regarding audio things).