You will need to locate the driver CD that came with your motherboard, or download the sound drivers for it online. Install those, and your sound will return.
As an alternative (or if you can't locate the drivers), consider installing Windows 7 instead. Unlike XP, Windows 7 can recognize quiet a few devices and install drivers for them. They may not be the most optimal ones, providing only basic features, but they will likely work.
Another reason to install Windows 7 is because it supports TRIM, which you will want if you ever install a SSD.