sound is not coming through headphone jack, only via built-in speakers on Dell Studio XPS.
After clean install of my laptop with Ubuntu 11.10 64-bit (Oneiric Ocelot), I no longer had sound through my headphones, only via the built-in speakers.
This resolved my issue:
Add this line at the end of the file: options snd-hda-intel model=dell-m6
Logout/login and problem is solved.
Experienced this issue already in 10.10 (if my memory doesn´t let me down)