The airline specifies what type of headphone jack are on the aircraft, so a 777 from one airline can be different from another. Generally you will find either a standard mini phone jack or a speciallized twin phone jack (one hole for left, one for right) You can purchase the twin plug adapters at Radio Shack or at a place that sells travel accessories. I have seen them at Office Depot. Noise cancelling headphones conveniently come with the adapters. So, with your headphones and one of the adapters you should be prepared for almost any airline's IFE system.

BTW, welcome to FlyerTalk.