Realtek and VIA are 2 different chipset makers. You have not provided any absolute brand and model adequate enough for us to help you. AC'97 is only a term created by Microsoft in 1997 defining sound standards. Almost all sound devices built from 1997 to 2007 are AC'97 compliant just like most all sound chips built since AC'97 are now HD (High Definition) compliant. Without any specific hardware description, all I can offer is to visit your pc makers website to see if they have a matching driver posted for downloading.
I answered a similar post just a few minutes ago that might address your problem: