I just manage to do this on my laptopt, using the mic input instead of line-in.
I did it by following the steps on this AskUbuntu question
I just had to download HDA Analyzer and run it as root:
wget -O run.py http://www.alsa-project.org/hda-analyzer.py
sudo python run.py
Locate the right PIN node (more details about it on askubuntu) uncheck IN and check OUT.
Click on export to save a python script that will make this same changes when run, then add it somwhere on your startup scripts.
I manage to get sound out of my mic jack, but it wasn't pretty. You'll probably have it better since you're using line-in instead of mic.