If your computer came with a motherboard disc the sound drivers could be on it
click start control panel administrive tools computer management device manager sound video and game controllers could be a yellow question or exclamation mark ! ? or red x right click to re-install drivers
cnetdowload.com if these drivers are not suitable on the home page you should see the download software finder under that there will be a list platform, category sub category price from the category drop down list select drivers then from the price drop down list select free or which ever applies to your need
http://download.cnet.com/Realtek-AC-97-Driver-Windows-98-Me-2000-XP-2003/3000-2120_4-10238712.html

http://download.cnet.com/Realtek-High-Definition-Audio-Codec-Windows-Vista-Windows-7-32-bit/3000-2120_4-10788600.html

http://download.cnet.com/Realtek-High-Definition-Audio-Codec-Windows-Vista-Windows-7-32-bit/3000-2120_4-10788600.html

http://www.freeware-downloads.com/soundmax_integrated_digital_audio_driver_v.htm
SoundMax Integrated Digital Audio Driver v hope one of these helps