I use a cable that I bought on line and it works ok but the volume in the aircraft comms system needs to be low to avoid swamping the Sony video camera that I use.
I also sit my camera on a full size tripod, two legs on the floor behind the seats and one on the rear seats, tied down, and this has the viewpoint in between the heads of the folk up front.
For two up flying this is fine, for solo I sit it on the right rear seat and it captures a front pax viewpoint.
As important as the sound, is the mounting or at least resting of the camera on a solid surface. Hand held video taken in an aircraft is not nice to watch.