Doing the Christmas holiday fix peoples computers bit. This one had no sound on youtube, but sound worked fine elsewhere. Flash video was fine, just no sound both in IE and in Firefox. I finally found my answer here although there’s a lot of information here. The links were annoyingly hard to find.
In the registry the key HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Drivers32\wavemapper was set to ‘msacm32.dvr’ not ‘msacm32.drv’. The youtube video that was playing in the background immediately started producing sound when I fixed the entry. Who knows what mucked it up. There are lots of stories about the Flash 9 upgrading causing troubles, I blame Norton and AOL, since they’re both on this computer and their bloatedness causes me nightmares about support problems.