The sound is not working at all! Help?

We have an IMac with OS X: The sound is not working at all. The icon at the top is greyed out and the sound adjustment buttons on the keyboard only display the slash through a circle symbol. We have restarted several times, reset the NRAM, and checked all the audio adjustments in system preferences. Stillnot working.

P.S.

We have tryied to put headphones in, but there is a pin looking thing in the headphone jack.. is that normal?(see picture) Does anyone know if it is possible to replace a headphone jack on an IMac?