How do Au'ra hear?

[MsQi]

Hearing different things doesn't matter so long as they translate to the same meaning at the end of it. It's like how you can't really prove people see the same colors, but they know that a color with certain RGB values is called "red" or "blue". Even if your red is closer to what I see as blue, for instance.
Assuming that sound translation is in effect when they speak too - so that when we need them to say "ah" they say "ah", even if what they hear is "oh" - then there won't be a problem of communication.

The design of the external human ear works with our brains for directional awareness. Sound from various directions has very slight changes (timing and frequency) due to the specific shape of our ears. There is variation from person to person but the same parts are there in most cases. Since they are all roughly the same, and leave a lot of the sound unfiltered we all hear roughly the same frequency response. What you hear also affects how you speak. Since the horns filter everything that come in then they have a greater effect on frequency response than our outer ears. The different horn shapes are likely to result in different emphasis on consonants. I suppose their society would adapt but to outsiders they would probably seem to have different "accents".