Sounds to me like you need a sound card driver for Win 7. If you plan on running 2 OS's, chances are you'll need 2 separate drivers for most of your components.
A driver is simply a piece of code that tells the OS and hardware how to interact with each other, so if the OS changes the driver must change too!
Install the Win 7 sound driver (while you are using Win 7) and all function should be restored! Good luck and let us know how it goes!