The main logic utilised for soundbending isn't that sound can only travel through air, it's more that airbenders routinely influence and shift air pressure in their bending style. Examples of this that can be seen include Zaheer's breathbending, which is by definition creating a region of extremely low internal pressure within the targets lungs since all air has been removed, and any kind of airblast seen is a rapidly moving, high pressure region of air. If it was low pressure, it would simply dissipate. Other elements have been seen to utilise pressure themselves (Earth with lavabending, Water with icebending), but those are more shifting pressure and then bending that, whereas Air systematically changes pressures. As a result, it stands to reason that an airbender would have a much easier time creating sound hypothetically than the other elements.