Quick google finds this (http://en.wikipedia.org/wiki/DirectSound);
Windows Vista features a completely re-written audio stack based on the Universal Audio Architecture. Because of the architectural changes in the redesigned audio stack, a direct path from DirectSound to the audio drivers does not exist. DirectSound and other APIs such as MME are emulated as WASAPI Session instances. DirectSound runs in emulation mode on the Microsoft software mixer. The emulator does not have hardware abstraction, so there is no hardware DirectSound acceleration, meaning hardware and software relying on DirectSound acceleration may have degraded performance.
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