My iPhone is playing music through one earphone louder than the other one?

I've had this problem before, and I solved it by changing a bar-like feature on the settings. But now I have iOS 7, it's gone back to the old setting and I can't find a way to change it back. I know people will think that I should just buy new headphones, but they can get expensive and thus I'd rather not.
Any advice would be great! Thanks: D
(p.s. I don't think it's just my headphones, but i'm not sure)

Have you tried… Settings>General>Accessibility… Down to Mono Audio then see if the L---R bar is central.