Click start, find my computer, right click it, select manage from the dialogue box, from the window that opens, select device manager & look under the sound video & game controller tab.
If there is an audio device or soundcard fitted to the pc & I'm assuming there is since you had sound before, it is likely to be listed there.
If as is most likely, you are missing a driver, it will have either a red line or a yellow symbol by it on the list. Click on it to open it & under the general tab of the device, windows will tell you its status. From there, you can choose to update driver or enable the device or do whatever it is that you need to do to get it working.
As it's a fresh install of xp, the overwhelming likelihood is that you need a driver, so be ready with either the utlilities disc or the internet connection to go and download one.