Headphones don't work in linux
 

I've looked up several pages to see if they can help me, but haven't seemed to find an answer.

My sound card is a HDA intel, chip: SigmaTel STAC9872AK

The sound works fine through the built in speakers but nothing comes out of the headphones.

Which Linux?

Have you tried the phones in a different jack?
Have you checked alsamixer?

I'm assuming this is a laptop, what make and model is it?

Check this page, specifically where it talks about the 'model=' driver option:
http://alsa.opensrc.org/Hda

I use Kubuntu and my machine is a sony vaio model PCG-384L. Does that help?

Alsamixer only has master sound, digital, and PCM. None of which affect my headphones. I have tried the phones in my microphone jack but as expected that didn't work.

Check alsamixer is using the correct soundcard.
Have you tried KMix?

Note PCG-384L is sometimes a VGN-FZ140 E mislabelled. Check.

http://ubuntuforums.org/archive/index.php/t-465491.html
... read through: headphone issue discussed.

It looks like the best thing to try would be using the model=vaio driver option. I think this is done in the *buntu's by adding "option snd_hda_intel model=vaio" to the /etc/modprobe.d/options file, I can't be 100% sure that is the file name, I've never used any of the *buntu's.