Why wont my macbook 4.1 black play music through my headphone jack?

It plays music and other sounds through my speakers just fine. As soon as I plug in any headphone into the port it goes dead silent and will not let me control the volume instead it shows the volume bar at full but it has a circle with a slash through it and will not change the volume thus leaving me in silence. I have played with the sound options but no matter what I click it simply does not change it. This is very annoying and I need my headphones to do my work.

My Macbook is a macbook 4.1 black. Its a square and not flat and rounded like the next models of macbook were.

I could really use some assitance on this because otherwise I just wasted my money on this product.