The architecture in Windows Vista provides the means to build and include custom DSP effects as part of an audio driver's value-added features. These effects are packaged as user-mode System Effect Audio Processing Objects (sAPOs).

There are three insertion points for sAPOs: pre-mix render, post-mix render, and capture. Each logical device’s audio engine supports one instance of a pre-mix render sAPO per stream (render LFX) and one post-mix render sAPO (GFX). The audio engine also supports one instance of a capture sAPO (capture LFX) that is inserted in each capture stream.

This white paper provides guidelines for device driver developers who want to create and install custom sAPOs.

http://www.microsoft.com/whdc/device/audio/sysfx.mspx