A3D was Aureal's environmental audio API. It was used by their Vortex cards.
It lost to Creative Labs' EAX standard only because, well, Aureal isn't around anymore to take care of it and Creative now owns what remained of Aureal.
In its time, A3D was much more sophisticated than EAX. It actually modeled most of the spaces as they were, when EAX only did much less interesting sounds! And even scarier fact was that most of the A3D lived on software - the driver had very tight, hand-coded, Assembly-language part. (Due to this fact Aureal probaly wasn't too eager to spread their card specs...)
(Some of the facts were as I remembered them, based on some Linux Journal article about OpenAL cooperation difficulties...)