I decided to install the ubuntu-desktop metapackage on top of my existing Ubuntu Server system. Since the tower is already hooked up to a KVM, I sometimes find it useful to sit down in front of it and use it as a desktop instead of connecting via ssh.
Unfortunately, sound did not work out of the box, despite the fact that ubuntu-desktop has dependencies on ALSA and PulseAudio. I was pleased to find that the solution was relatively easy. I only had to fire up alsamixer, unmute PCM, and raise the volume. My next task is to get Music Player Daemon (MPD) working.