Hearing actually is stated as being quite a rare skill in Gridania, and not one usually considered a 'standard' ability of conjury - there are plenty of CNJs around but actually nowhere near as many hearers. And naturally, padjal hearers are even rarer. It seems to come down to sensitivity of one's surroundings, which is the key to communing with the elementals.

The CNJ quest line goes into that in detail (it should be noted certain hearers seem to possess a certain strength when communing with a particular elemental - the hearer Wulfie at Rosa Hawthorne's hut in the Honey Yard for instance specializes in communing with the element of water for instance - she possesses a particular sensitivity in that particular element), particularly concerning Sylphie's latent sensitivity to the elementals - she just had to let go of her emotional baggage and stubbornness before she could do that.