iPhone 4s only producing mono sound
The fact that people say they are getting sound from both grills at the bottom of any iPhone is total nonsense, all models have a Microphone and a speaker at the bottom.

If Apple replaces the microphone with a speaker, the phone would be rendered useless as there would be no way of it picking up speech.

It is a very common misunderstanding. 

A couple of things to check.


Clean the headphone jack - any dust or fluff can cause the signal to break
Go to -Settings>general>accessability>Mono audio make sure this is off
Compare with a friend's phone, same model, same track, same head phones, then try new headphones, and after that a new song.
if there is still a problem go to an Apple store.