Does air bending have a sub skill, and if it does what is it?

Each sub-skill is a relatively rare skill compared to the population of that type of bending that requires special training (with one exception) to learn. As the population of air benders has been tiny and many nuanced details of the form have been lost over the centuries. Aang was a good airbender but likely didn't know everything as he was a child. From this we can speculate that if there was one, it may have been lost.

I have not found an example in Avatar nor Korra but I'm a little behind on Korra (which is why I didn't answer quickly).

We can speculate many different interestng ways manipulating air could be specialized for different uses: sound bending, foam bending, breathe stealing, rapid oxidation, elemental separation, forcible phase transition (that could get gory), etc. This, however, would not have any canon support.