In this previous post, I mentioned that everything was working correctly after upgrading to Ubuntu 8.04. Unfortunately, after I wrote that post my audio stopped working.
I went through several steps to get audio back, including building and installing the latest version of alsa.
I found that if I put the following line in my /etc/modprobe.d/alsa-base file, everthing works
options snd-hda-intel model=dell-3stack