Apple - MacBook Pro won't recognize my analog audio jack headphones. Help?

There seems to be a bug whereby if the headphones are plugged in when the MBP is put to sleep and are not present when it is rewakened, the headphones stop being recognized. The solution is to:

1. Unplug the headphones
2. Put the MBP back to sleep (close the lid; without an external display hooked up)
3. Plug the headphones back in
4. Wake the MBP by opening the lid.

This should fix the problem.

Ok, looks like I fixed my issue by resetting PRAM. Here are the steps:

1. Shut down your Mac.
2. Locate the following keys on the keyboard: Command (⌘), Option, P, and R. You will need to hold these keys down simultaneously in step 4.
3. Turn on the computer.
4. Press and hold the Command-Option-P-R keys before the gray screen appears.
5. Hold the keys down until the computer restarts and you hear the startup sound for the second time.
6. Release the keys.

Source - http://support.apple.com/kb/ht1379

open system preferences and go into sound. under the output tab make sure where it says "use audio port for:" is set to "sound output"