In my Sound preferences i have two outputs: Headphones and Speakers. When i plug headphone jack, it switches to Headphones and back to Speakers on unplug. But if i manually switch output to Speakers, signal on headphone jack appears immediately. That is it is the same output in hardware level. I think, i need to reconfigure by hands any sound subsystem or something else, but don't know exactly what. Don't sure this is bug, but a little part of quest named Ubuntu. It happened fatter my upgrade from 10.04 to 12.04.