I pass this along because I know how much people here value the sounds that come from their computers. My girl friend just reformatted her hard drive and arrived at the point where she decided to get windows update (XP). When it had installed, she lost her sound. She told me about this and I thought it was just a little odd. I Googled on "windows update lost sound" and discovered pages and pages of people with the same problem. She was able to restore her sound with a restore but has bypassed the current update.
It could be a lot of things. Since a system restore did the trick, my hunch would be a driver conflict. Is an update to the sound card in the list of updates?
If she updates and loses the sound again, you might see if something else has been set as default in the Device Manager dialog accessed from the control panel, or if rolling back the driver does the trick.
I updated WinXP SP3 Monday - no problems. However, you did mention a hard disk reformat, which may be involved in the problem.
If she has integrated sound, hit F2 while booting up to check integrated peripherals to see if the sound is turned on. If she has added a sound card to her system, the integrated sound should be turned off to allow the new sound card to work properly.
If the problem persists, checking the Windows forum might be helpful, as sometimes you'll find answers to update problems experienced by a number of people. I recall about a year ago that an update caused an internet connection problem, which turned out to be a problem with my Zone Alarm firewall not being prepared for the update for several days.