Finally I did it. It was a combination of several issues regarding the Cirrus hardware and drivers deliverey by Apple with the BootCamp package for x64 architecture on the Snow Leopard DVD and the newer generic drivers provided by Windows.
Here in short what I've done:
- uninstall the (not only deactivete) the Cirrus device on Windows 7
- Reboot (and do not install the device, if don't do so, Windows 7 will install a generic driver provided with Windows)
- Install the Cirrus device by and select the latest drivers from Cirrus