WASAPI

Vista has a completely new audio mixing engine, so WASAPI gives you the chance to plug directly into it rather than going through a layer of abstraction. The reasons for the new audio engine are:

• A move to 32 bit floating point rather than 16 bit, which greatly improves audio quality when dealing with multiple audio streams or effects.
• A move from kernel mode into user mode in a bid to increase system stability (bad drivers can't take the system down).
• The concept of endpoints rather than audio devices - making it easier for Windows users to send sounds to "headphones" or record sound from "microphone" rather than requiring them to know technical details about the soundcards installed on their system
• Grouping audio streams. In Vista, you can group together all audio streams out of a single application and control their volume separately. In other words, a per-application volume control. This is a bit more involved than might be at first thought, because some applications such as IE host all kinds of processes and plugins that all play sound in their own way.
• Support pro audio applications which needed to be as close to the metal as possible, and keep latency to a bare minimum. (see Larry Osterman's Where does WASAPI fit in the big multimedia API picture?)

Source: Mark .Net