I think that this is a short inside the headphone jack.
This is occurring because of the extra pins (as seen in Hatty's pic). The socket on the mac is designed for one with 3 pins, not 4.
Therefore, the same effect as this is produced, actually using the same idea as that.
You would need to find an adapter or replace the connector (on the headphones). If you want to replace the connector then follow this guide.
Alternatively, you could get a adapter cable made (or you could try to find one).
Does anyone on here have the electrical skills required and would be willing to make an adapter?
I am only 13, and can just about use a soldering iron, but I'm really not that good.
EDIT: This might do the trick?
Quoting Hatty ;)
P.S. This is my opinion, and I don't know if it's correct or not!