First open your device Manager and uninstall your audio driver and reboot.
in your run box this will open device manager find your audio device right click it then uninstall. Reboot your machine.
Try this program to get an update for your audio driver. DriverMax - free driver download program for Windows XP & Vista
You need to register the program in the link above and you get two free drivers in a 24 hour period.
Also take a look at the links below. Tips for fixing common sound problems http://windows.microsoft.com/en-us/w...und-in-windows
Please post the log created by the program in the link below. Unknown Device Identifier - Freeware Download
Hit start button at lower left hand corner of the screen.Then in the run box type services.msc. Look for windows audio/anything to do with audio and make sure the startup type is set to automatic. To do this,right click selected service select properties then change the startup type to automatic then left click apply.You will need to reboot your machine for the changes to take effect.