Why only one side to headphones/earphones/speakers work with my iPhone 4S?

When it started playing music/video, only out one side of the head/earphones or speakers. I know it's not the earphones cause I've tried it on other devices (other phone, my laptop) and even when I use another earphone that is perfectly working, it only comes out one side on my iphone. So I'm pretty sure that the problem is from the phone.
I've also tried Settings>General>Accessibility, look for the left<-->right slider under, but no luck.

Does anyone know how to fix this?

The phone might be wired up with a mono jack

Lint build up in the audio port could cause this as well. It would keep the earphone jack from plugging in all the way. Check for lint build up and if there's, get a needle and fish the lint out but do this very carefully to ensure no permanent damage will be caused. I've had to do this a few times due to wearing blue jeans on the weekends. - Hunter