EAX is an interface for 3D sound, used by several PC games such as Angel of Darkness. It enables these games to play back sound as realistic as possible e.g. by distributing noises to the corresponding speakers of a 5.1 surround system.
EAX and Windows Vista
In Windows Vista the Direct3D interface of DirectX, which is used by EAX, has been removed. Since AoD uses solely EAX - while installing AoD it is automatically installed, too - it won't work any more.
Creative has created a project, that develops a program named ALchemy, which routs the output of EAX to OpenAL instead of Direct3D. Both interfaces can produce the same effects. Thus it is a so called "wrapper". (Similar to Glidos, a Wrapper that redirects the old 3dfx-Glide-commands of TR1 to DirectX in Windows.)
However OpenAL is at the moment only supported on Creative X-Fi sound cards, and it is uncertain if there will ever be some kind of software (not dependent on hardware acceleration but OpenAL implemented within the sound driver) for other sound cards like it has been with EAX and Direct3D. Until then you need Windows XP/2000 to play AoD if you don't have a X-Fi card.