Or, just stick a regular 3.5mm headphone jack into the two-pronged hole half-way in. The contacts line up in such a way that one channel touch both inputs. Just turn up the volume a bit.
http://www.ebay.com/itm/2-pcs-3-5mm-Airline-Airplane-Earphon...
In most airlines the announcements are at fixed volume, independent of the movies. That means you can put the video on full volume and turn it down inline. No more announcements screaming in your ear :) (and better sound quality as a bonus)