Solved! My headphones are louder on one side than the other.

[brookcrane0603]

This doesnt happen for other headphones, and the balance is set in the middle. There is sound coming out of both sides, one is just way louder.

 

[v12vantage]

Obviously check the jack is seating into the socket fully but apart from that It sounds like a problem with the headphone jack on the end, they are surprisingly delicate. In my car I had to replace the AUX lead because it was slightly bent which caused the sound to go out of phase (a bit like when you connect up a pair of hi-fi speakers and get the positive and negative around the wrong way).