I use VirtualBox for my virtualization platform on my laptop (sorry Microsoft).  This allows me to have on-the-go servers for demo and lab purposes easier than any other product.

One of the main products that I use is my lab is Lync server.  VirtualBox has no issues when installing audio drivers for Windows XP clients, but it does not install audio drivers for Windows 7 clients. 

Lync client on Windows 7 shows no Audio Device:

VirtBox - Lync no audio - markup

Of course looking at the Device Manager shows that no audio driver is installed:

VirtBox - Device Manager no Audio - markup

This is a big issue for me since I want to be able to test/demonstrate voice and conferencing within my VM environment. 

You can install the driver manually by downloading and installing the Realtek AC’97 Audio driver found HERE

VirtBox - Device Manager driver

After installing the driver, my Lync client now shows a valid audio device:

VirtBox - Lync with audio - markup

I can now successfully make phone calls within the VirtualBox environment:

VirtBox - Lync call

Hope this helps you out if you are also using these tools.