Hi
Here's what I have found previously. If you plug in your own headphones into just a single socket (which you can do), then usually you find the volume hard to control. That is, at base level, the volume is very loud, and when ccrew make an announcement, you wll be deafened.
Recently, I got a pair of those snazzy bose quiet comfort noise cancelling headphones, and in the box, there is a cleverly designed adapter, which can use one or two sockets. One of the jacks is a standard size, whilst the other is smaller. This is explained by the instructions, that the second smaller jack, when needed by an airline seat, controls the volume.
So from what I gather, you can plug into a scket with two jack points (one bigger than the other), with a normal headphone jack, but will have less control of volume. The preferable scenario, however, is to use an adapter that will enable you to plug into both, the standard size hole, and the smaller sized hole.
I think?