Thief will be shipping with A3D 1.x and DS3D support. EAX support will be added in a patch and we are still trying to get word on A3D 2.0. The Dark Project and as such, it has really captured my attention from the get go. Well to stall a moment longer before getting into my impressions of the demo I want to quote some notes from Looking Glass to set the stage. These are from the various support documents that accompany the demo.
Thief supports a lot of exciting features that are available with more advanced sound cards.
Sound plays an important role in Thief. When you're sneaking around some guy's house at night, you want to be able to hear which direction the armed guard is coming from, how close he is, and what sort of surface he's walking on. If your sound card supports hardware acceleration using DirectSound3D, we strongly recommend that you use it while playing Thief. Thief is not one of those games. Of course, the best way to survive a fight is avoid it in the first place.