I recently installed Windows 7 in VirtualBox, and unlike other OSes I’ve installed in VirtualBox, the sound driver wasn’t automatically recognized. I found a driver that works with both 32-bit and 64-bit versions.
I installed this both before and after installing the Guest Additions, but it didn’t matter what order the driver and Guest Additions were installed in.
Enjoy. 
These days, I really want to dabble around more with virtual machines. They have a lot to offer for benchmarking and failover protection. However, I always thought that you needed a host OS to run the virtual machines in. In my experience, this was clunky at best and riddled with problems such as automatted startup of virtual machines, stability of the host OS, and degraded performance.
I’ve used virtual machines lately with Sun’s VirtualBox, a free virtualization platform that can run virtual machines. This has been valuable for testing different distros, but wouldn’t suffice for running virtual servers. At least, it wouldn’t based on my experiences.
I have a friend that has worked professionally with virtualization technology for a few years, and he pointed me to a VMware product called ESXi. Not only is ESXi free, it runs as the native OS in which other OSes can run virtually.
I have yet to test it out, but it shows great potential. If you have any experience with ESXi, I’d love to hear it.