Have you attempted to install the audio drivers for your sound card? Also, have you ensured that your sound card is indeed compatible with Windows 7?
To verify compatibility for hardware and software concerns you can always refer to the Microsoft Windows 7 Compatibility Center available at the following link: http://www.microsoft.com/windows/compatibility/windows-7/en-us/default.aspx
Also, Microsoft does have an official Windows 7 Support Forum located here: http://social.technet.microsoft.com/Forums/en-US/category/w7itpro/.
It is supported by product specialists as well as engineers and support teams. You may want to also check the threads available there for additional assistance and guidance.
Microsoft Windows Client Team