I don't know what caused the sound to stop working (WebCam and/or Vista updates, who knows?) but it stopped. I assumed that it was something simple but couldn't find anything obvious and Googling didn't come up with anything particularly useful. Adobe's knowledge base talks about needing device drivers and suggests re-installing, and someone else suggested selecting the appropriate sound device via Window's control panel. While investigating I discovered that clicking the ASIO Settings button on the Edit > Preferences > Audio Hardware dialog caused Premier to crash without exception.
I did re-install to no avail and then in desperation started digging deeper and noticed that the volume control mixer was showing Premiere in the apps section of the digital (S/PDIF) audio device (I don't have a digital audio device hence I couldn't hear anything). As a clip played I could see the green level meter move. Because there didn't seem to be anything I could usefully change in Premiere I tried disabling the digital (S/PDIF) audio device via the Windows Control Panel (Sound > Manage Audio Devices) and this solved the problem. The sound and the ASIO settings... button started working again. Subsequently re-enabled the digital (S/PDIF) audio device didn't cause any problems.
I've submitted this in the hope that it will help others. Adobe's knowledge base and help seems particularly poor on this subject. Even searching this forum for "no sound" results in absolutely nothing. Interestingly, searching for "sound" also finds nothing which I can't believe. There seems to be something fundamentally broken with the search. Searching for "mpeg" came back with results so I can't believe its got anything to do with using a single word and there being too many results. Even if it was it'd far more friendly to say so. The whole experience has been very unfriendly and doesn't endear me to Adobe, its products or its forum.