I don't know if this problem happened when I installed Ubuntu before.
Recently I noticed that when I boot Ubuntu, the Digital Audio Output light automatically switches on. Digital Audio Output light on means "Something wrong in the headphone port". Although my headphone is working in Ubuntu.
I've heard that the headphone contains some magical "switch" that will fix the light problem. So I poked the headphone port with chopsticks, pens, paper clips, even my finger, and the Digital Audio Output light still stays on.
I don't have this problem in OSX.
How do I switch the light off?