Windows 2003 Enterprise edition comes with the sound disabled and virtual PC uses the sound blaster 16 driver to produce sound.

However, the sound blaster driver is not on the windows installation disks and is no longer available for download on the site of Creative. A workaround is to copy the files from Windows XP and install those. See the blog of Virtual PC Guy for more info. (Just copy the files below in a folder and point the add hardware wizard to them.)

For convenience I have zipped the files so you can download sound blaster 16 drivers here.