I know this is a very old thread, but I have found the most likely answer to this problem.
I've had i-products since the scrollwheel gen 2 iPod, and with every single device, after some time using the earbuds, the left ear has gotten quieter and quieter. It just happened to my iPhone 4, which lasted longer than any of my older ones (since December 2010), so I went searching to find other people who have complained about this.
Ok, so the earbuds themselves have their speaker behind that little mesh-like thing that directs into the ear. On the back of each earbud, there are several little holes. I'm not an expert by any means in audio, so I have no idea what their purpose is.
Anyhow, several of the little holes had crud - dirt, dust, earwax, who knows - in them. In the left ear I had three of the little holes blocked, and in the right ear only one. I guess it's because these earbuds fit perfectly in my right ear but not my left. So I decided to clean them out, give that a shot. Using one of my previous pair of earbuds with the same problem, I took the quick way and used an air spray can. Once the holes were clear, sound was back to normal, same volume in both ears.
Probably smarter to use a thin needle or pin to scrape the dirt out instead of blowing them inside like I did, but regardless, it worked.
I hope this helps someone, being as old as it is.