To get HDMI audio working on my lucid machine, I added some PPAs and then did:
$ sudo apt-get install linux-alsa-driver-modules-$(uname -r)
That fails currently, since the PPA hasn't kept up to date with the most recent kernel:
E: Couldn't find package linux-alsa-driver-modules-2.6.32-34-generic-pae
So for now, I've just downgraded to the old kernel. I also removed linux-image-generic so that it wouldn't automatically upgrade my kernels in the future:
$ sudo apt-get remove linux-image-generic-pae
$ sudo apt-get remove linux-image-2.6.32-34-generic-pae
Update (31 Jan 2012): Good grief, and now it's broken again. The latest kernel is 2.6.32-38. The fix this time was to add the lucid-proposed repository and install linux-backports-modules-alsa-lucid-generic-pae. That installs linux-backports-modules-alsa-2.6.32-38-generic-pae, which seems to take the place of linux-alsa-driver-modules-2.6.32-38-generic-pae.
To add lucid-proposed in synaptic, it's Settings... Software Sources... Updates... Pre-released updates.
That adds this line to the /etc/apt/sources.list:
deb http://us.archive.ubuntu.com/ubuntu/ lucid-proposed restricted main multiverse universe