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.

ftp://202.65.194.211/pc/audio/

This is a mirror site used by Realtek. Note that the download speed is kinda slow, so be patient.

[Update: 11/16/2009]

The ftp site is protected by password now. It’s not working unless you know the username and password.

[Update: 4/4/2011]

If you have problems following the link, please try to use Internet Explorer to download the driver.