According to Larry Osterman, Microsoft Principal SDE, Windows 7 will not feature an overhauling of the audio subsystem, but the platform will indeed come with a range of enhancements including improved device role detection, Bluetooth audio in-box, stream switching, improved audio latency, automatic attenuation of audio samples (ducking), etc.
“The first thing we did [in Windows 7] is what we call improved device role detection,” Osterman revealed at PDC 2008. “And the idea behind this feature is that we want to set things up so that when you're a multimedia application and you want to use audio, it will be really nice to know which device the user has chosen to use.”
“Using the PC as a phone is central to the future of communications, and this session will provide you with the essentials to build the end to end experience. Windows 7 provides APIs for integrating communications capabilities into your applications. This session covers attenuating and muting sounds during a phone call, receiving a phone call using a Bluetooth headset, improving the latency of a phone call, and selecting the appropriate communications device,” reads the abstract for Osterman's session at PDC 2008 available for download here and for viewing here.
Inside Windows 7: Larry Osterman on new audio capabilities