Having recently updated my OS to Windows 7 64-bit, and using the default audio drivers that came with it, I was pleasantly surprised that (other than enabling the volume from the mic) no particular configuration was necessary to enable audio pass-through.
However, I soon realized that there was a consistent hissing sound. Initially reluctant to install 3rd-party drivers into what has been a vanilla and generally very stable Windows 7, the hissing eventually irritated me enough to take the plunge and install the generic Realtek High Definition Audio drivers (in this case the R243 version).
The 50+ Mb Realtek executable file installed perfectly, silently removing the Microsoft drivers and prompted a reboot. Upon reboot, I opened Control Panel -> Hardware and Sound -> Realtek HD Audio Manager, and enabled "Playback Volume" for the microphone.
Viola! The hissing was totally gone, audio passthough works without a hitch.