I have made some drawings of the current Wine audio architecture, the WinXP architecture, the Vista architecture and the proposals we have seen. The drawings are based on information which can be found on MSDN. Regarding Vista/Win7 it would be useful to read: http://msdn.microsoft.com/en-us/library/dd316780%28VS.85%29.aspx which explains the architecture in detail. Basically all Microsoft APIs use WASAPI which in turn implements IAudioClient and other classes which talk to an 'audio engine' which is implemented in audiodg.dll/audiodg.exe. This audio engine is quite similar to pulseaudio and it offers functionality like per stream volume which normal APIs like oss/alsa/openal don't offer, so these APIs would map better to a sound server than to a standard library.
Roderick