Sounds like the micro switch inside your output jack maybe stuck. Try plugging in a set of buds or headphones and see if you are getting sound from them. If so you might get the micro switch to unstick by jiggling and twisting on the headphone plug before removing it.
You should also, go to: Applications > Utilities > Audio MIDI Setup and check the Output section there to see if perhaps someone or something has changed some settings there.
If that doesn't work then contact your local AASP or Apple Support and make arrangments to have it looked at.