Originally Posted by AlphaWolf_HK
Correct, except for the games and EAX part. Games are with very few rare exceptions entirely discrete audio. Sound in games consists of just numerous mono sound clips, and those clips are mixed into the individual channels in real-time.
Many game engines only do this with two channel support built into the engine. Most that don't use directsound to process this, the game just tells directsound where the individual sound effect is coming from relative to the "listener," and then directsound automatically takes care of the channel mixing. The same is done with surround as well, with directsound3d. Both directsound and directsound3d use discrete audio; that is they can send completely different audio to each different speaker if desired.
EAX actually has nothing to do with surround processing (aka positional audio) at all. EAX, meaning Environmental Audio eXtensions, is literally just a proprietary set of extensions for directsound3d (and optionally, OpenAL) which add various types of effects (e.g. reverb, echo, chorus, envelope, etc) to a given sample to give it an effect to make it sound like a different "environment" (e.g. hallway, stadium, etc.) EAX AdvancedHD extends EAX a little further by being able to apply multiple effects to a given sound sample, as well as add transitional effects to a sample which e.g. comes from a different type of room than the listener is currently standing in.
With regular directsound3d, in say for example the arena in Oblivion, you can hear those footsteps coming at you from whichever direction, be it front or rear. With EAX extensions, it can sound more like you are actually inside of the arena instead of just your room because of the echo effects that are added. With EAX AdvancedHD, this would sound the same as regular EAX unless you were down in that hallway before you set foot into the arena, and heard something moving in the arena, in which case you'll hear sounds coming from the arena into the hallway where you are standing. Without AdvancedHD you just hear the sounds as if they were from far away in the hallway, with no "distant arena" effect added in. (I don't know if Oblivion uses EAX BTW, this is just a hypothetical example.)
Also the differences between eax 1.0 through 5.0 (aside from AdvancedHD being added in 3.0) are only changes in the maximum number of audio samples that can be processed at a given time.
(and off topic a bit, FWIW, vista doesn't remove eax nor directsound3d as many fud spreaders claim, it just removes HAL support which tends to be buggy, and this made creative freak out because now their marketing team has to change its tactics)