During the second season finale, Aang had to open the seven chakra seals. The first four related to the four elements, and the last three were the, the sound, light, and thought chakra.
Granted, this is only my interpretation, but I saw the last three as different forms of bending. The thought chakra, at the head, reminded me of energy bending, as Aang always has to touch a person's forehead to give to give or remove bending. The light chakra reminded me of lightning, it's not actually fire, but created by heat. Blood, ice, and steam are just water in different forms, there's no new bending here, as well metal is still just refined earth.
So when I think of sound, I think of it as a possible extension of air. It's still related to the atmosphere, but like fire, it's different from it's first form, but not unrelated.