If you're sure you don't have the sound drivers installed,then you can get them from your Motherboard's DVD. If you've missed it,you can download the latest drivers from your Motherboard's manufacturer's website. For that,you'd need to know your MB manufacturer and the model of your board. For example,if your MB is from Asus,you can get the drivers from the Asus website:
http://support.asus.com/download/options.aspx?SLanguage=en&type=1
Like that,each manufacturers has their own websites,in which they provide the latest drivers for sound,video,Network and so on....
And,if you have a sound card added to your system,you'd need to download the drivers for the card,and not your Motherboard. For doing that,just google for the drivers with your sound card model....