The quickest way to bring up the Device Manager is:
Start -> Run -> "devmgmt.msc"
If it detects the sound hardware but there is a problem with a driver, you will likely see a yellow "!" appear by your sound device.
If it was working and suddenly stopped, you might try performing a "System Restore" to a point in time when it was working:
"How to restore Windows XP to a previous state"
< http://support.microsoft.com/kb/306084 >
HTH,
JW