Try cutting the end off a Q-tip, and inserting it in the headphone jack, and GENTLY "massaging the jack to free up the micro switch that sometimes gets hung up. You can also try plugging in and out a set of headphones several times to free it up and return it to analog out put. Other fixes are a SMC reset, a pram reset, putting to sleep with phones plugged in removing and waking. These are the most common, although there are a few more less common fixes, try these first then repost if no joy. Hope this helps.
Click battery you can remove yourself, or battery you should not remove yourself link, and follow instructions.
Power down, power back up holding the following keys: option, command, P, R, (no commas), continue to hold these keys till you hear the start chime time 2 times then release. Reboot.
See if this helps.
SMC and PRAM does not control the sound so you can not reset it. It's obvious that if you plug in the headphone and it works but once you unplug it no sound that it's a settings issue within the Sound control panel. Go to System Preference-->Sound. Select Output tab. and see where the sound is outputting to. It should say Internal Speakers if you unplug the headphone.