This seems to have worked for me. http://forums.virtualbox.org/viewtopic.php?f=3&t=25444
In short, I used vim to edit/create '/etc/modprobe.d/sound.conf', which for me didn't exist.
Then, in the file, paste this code:
sudo vim /etc/modprobe.d/sound.conf
Finally, reboot. (I'm assuming restarting sound works to, I didn't try it though. YMMV.)
options snd-intel8x0 ac97_clock=48000
options snd slots=snd-intel8x0
# CvwD.FAMlirE10w6:82801AA AC'97 Audio Controller
alias snd-card-0 snd-intel8x0
FWIW, My host is Mac OSX 10.6.2 running VirtualBox 3.1.4, Guest is Ubuntu 9.10.