When I installed Windows 7 Beta (Build 7000) on my early 2008 MacBook yesterday, the audio driver did not work. The driver provided by Boot Camp is not the right one. Using the hardware id, I learned that the audio card is made by Realtek. I downloaded the driver directly from Realtek and it worked. Here is the download link. The one I downloaded is Vista Driver (32/64 bits) Driver Version R2.14. Extract the files and run the setup.exe.
BTW, if you are looking for some older versions, you can use this ftp site.
This is a mirror site used by Realtek. Note that the download speed is kinda slow, so be patient.
The ftp site is protected by password now. It’s not working unless you know the username and password.
If you have problems following the link, please try to use Internet Explorer to download the driver.