I just encountered this problem having previously resolved audio issues via the OS I needed to replace the audio cable this time. Easy enough fix if you're willing, took about 20 minutes once I had the part.
I examined the cable I replaced and could see nothing obvious although I did note that the cable doesn't quite sit as nicely as I would have liked, it is possible this will cause stress to the cable over time and the problem will reoccur. But given that I have has the machine since it was first launched (I forget exactly when that was) it doesn't worry me too much.