Well technically we do know that it's soundebnding..
One time Aang used airbending when he blew into the Bison Whistle to make it sound really loud. But I think he was really just making more air pass through it so... I dunno. Still.
...because it's air? confused
What else can you do with air?
I'm convinced the "special" talent is probably making a person unable to be breathe, but airbending is about evasiveness. Also, the Air Nomads were a peaceful people. They're not likely to seek out ways to mess with the air like that.
I'm pretty sure air doesn't need a second thing. The other elements have them because they are all part of the element. Blood is just like water, metal still has earth in it but only a truly great earth-bender has the power to bend it, and lightning... I have no idea. Air is pretty much just something by itself. Unless air gliding is one.