The problem: Windows XP suddenly stops playing back sounds. Drivers are present, sound devices menus all appear correctly and the volume indicator might even be present.
I first tried just changing settings around to see if it had been an error caused by a human. No dice. I uninstalled the driver and software. In this case, SoundMAX. The driver available from Lenovo was from 2005, but has not been updated since. Installed, but still no dice.
Finally I started thinking about audio codecs and ran a search for “K-lite codecs” and found the cnet link. I chose a custom installation, that also included a registry check on the sound system. Sure enough, it worked!