I'm having the same issue on Mac OS X 10.6.1. (VB v3.0.8 / .10)
My previous install of Win7 beta had sound working fine (VB v3.0.
; I upgraded to a real copy (Home Premium) and installed the 64-bit version, which broke the sound, though everything else was fine -- so, I followed all the instructions in these forums, got the drivers from realtek for the AC97 soundcard (which said explicitly they were for vista/win7). Unzipped the file, and saw there are Vista and Vista64 folders in there, which was a good sign, but running setup.exe launches the installer and ultimately tells me "Only Vista is supported with this release." I tried installing the drivers manually from Device Manage / Update Driver / Browse for drivers, but it can't see any valid drivers. (Also tried mucking about in the setup.ini files to see if there was a way to hack out the OS version check, but didn't see anything obvious.)
I then updated to VB v3.0.10, with the same results.
So, I can't even get to the point of telling Windows to search for an upgrade.
Seems like it's specific to the 64-bit version, that's causing the problems for me and others? Has anyone found a workaround? Loved the comment that this whole ecosystem is still so broken. Makes me smile when I think of the new commercial: "There won't be ANY of the problems of Vista... I promise." Heh. Well it's a fun little experiment.
(Otherwise, very impressed with VirtualBox -- been using it for about a year. Fun to show off to all my PC-centric workmates!)