Hmmm... That happened to me too. I had to throw it away, as you may know that some of the music you are listening to comes from the left, and the other from the right. Here is an example. You will understand at 0.44 secs if you are using headphones. I would advise to throw it away/fix it, but that is so annoying. I bought a new one, and it works better.
Well when this occurs, some wiring inside is damaged due to too much strain on it. It can be caused by too much pulling on it or yanking on it. Of course, it's not necessarily on accident or purpose; this can be due to wear and tear.