After installing Ubuntu 10.04 on a Asus EEE 1005PX netbook, the audio output jack did not work. When headphones were plugged in, the netbook speakers were cut off, but there was no sound at all in the headphones.
The solution was to add a line at the end of file /etc/modprobe.d/alsa-base.conf :
options snd-hda-intel model=lifebook
After the next reboot, the headphones worked perfectly.