If you have a macbook and there is no red light in the sound output jack, your audio board could have a bad cable or the board my have gone bad. If there is a red light on inside the jack, your jack is stuck "on". You can stick headphone jack in and out rapidly to fix it. You can also use a toothpick. The apple store will replace the board and cable for a fee as there is no repair on ifixit.com as of 11-5-10. My research has led me to this, hope it helps.