Audio Line In/Out not working

Are you sure this is not your headphones? I've had many pairs that went bad from tension being put on the jack. Make sure they are "stereo" headphones, if you're using monophones then you will only get sound in one ear no matter what you do. Or try plugging in external speakers. Anything to rule out the headphones, and making sure that you are using a compatible pair.

If not, then you should try powering down your Macbook, reset the PRAM/NVRAM.

If that doesn't work, then try powering down, unplug then reinsert your headphones while it's off, then power it up and see if they are working, this is a typical fix for older macbook pros.

If that doesn't work, go into your system settings and click on audio, make sure your software is set to "stereo" and when you insert your headphones it switches to line out under audio output.

If that doesn't work, and you're positive it's the audio jack, remove the bottom cover of your macbook pro, check to ensure the audio jack has not been broken loose from the logic board. if everything looks good at this point, you may need to just replace the audio jack. It's not hard and I would post a tutorial but I've never had to replace it. If you see a broken connection, just resolder it, it's pretty easy just take your time and have a little patience with unsoldering and resoldering your logic board, make sure not to burn it.

If that doesn't work, check the "balance" option under sound in system preferences.

If that doesn't work, go into your audio midi setup in applications/utilities, play with the balance by checking unchecking each headphone and see if you can get sound from both channels left and right.

Check accessibility options under hearing I believe, there is an option that controls sound coming from only one side of the audio out.

Hope this helps!