The Free On-line Dictionary of Computing (30 December 2018):

A3D

    (Aureal 3-Dimensional?) A technology developed by
   Aureal that delivers sound with a three-dimensional effect
   through two speakers.  Many modern sound cards and PC games
   now support this feature.

   A3D differs from the various forms of surround sound in that
   it only requires two speakers, while surround sound typically
   requires four or five.  It is sometimes less convincing than
   surround sound but is supposedly better in interactive
   environments.  For example, PC games in which sounds often
   move from one speaker to another favour A3D, while
   pre-recorded video favours surround sound.

   (http://a3d.com/).

   (1999-01-26)