In Vista, I have seen my network and sound icons disappear from the notification area quite often. Microsoft KB article 945011 explains why and how to workaround the issue. Unfortunately, this isnt a permanent fix and they still seem to disappear occasionally. I have created a batch file to do the work for you so that you can run it often if need be. Take the following text, paste it into a text file and then save it as restore_icons.bat or something similar (must end in .bat). Run it to restore your icons.
NOTE! This blog breaks up the first two lines at the first space. Make sure you piece the code back together as it should be.
reg delete “HKCUSoftwareClassesLocal SettingsSoftwareMicrosoftWindowsCurrentVersionTrayNotify” /v IconStreams /f
reg delete “HKCUSoftwareClassesLocal SettingsSoftwareMicrosoftWindowsCurrentVersionTrayNotify” /v PastIconsStream /f
taskkill /F /IM explorer.exe