How to give input to microcontroller from a 3.5-mm stereo jack
Here is a video of exactly what you want (I think,) Arduino Tutorial: Led Audio Meter via HeadhPhones

If you just want to know when sound is there, not actually process the sound, then all you need to do is use an ADC pin. 

ADC is just an analog to digital converter, you can learn more about that here. The resistors in the picture below are used as a voltage divider to prevent >5volts from getting to the MCU. You may not need them at all since according to this the voltage shouldn't get that high.

You could wire it up like this, except the input voltage would be the stereo wire + and ground (if it's stereo you will have a left + and right +, you can hook them together if you don't care about the sound, if you do care about the sound then use one pin for left and one for right.)


Here is a link to the whole setup: http://arduino.cc/forum/index.php/topic,27223.0.html