Microsoft actually removed the hardware sound acceleration from Vista entirely, all sounds in Vista are processed by the CPU, and this means a lot of hardware-accelerated effects (like EAX) aren't supported anymore.
The only way to get hardware-accelerated sound in Vista is to bypass DirectX and go with OpenAL, but very few drivers have been updated yet (and very few games too). Creative's approach is to try and redirect DirectX sounds to OpenAL, to get hardware accelerated sound again in old and existing games, but that's still quirky for now.
If you don't have a high-end dual-core CPU, until every ones supports OpenAL, you may probably want to reduce sound quality and turn off 3D sound in Vista, or your framerate will suffer.
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