Sound problem in acer laptop

respected sir,
i do have the problem with my Acer aspire 4720z that its sound id not working at all.the video is being shown up but sound is not coming by neither headset nor speaker.when i am installing my audio driver then it gives the message that realtek hd audio driver failure.i formatted the system and then tried but the same message.i tried to download and install also but it gave the same message.please show me the way sir.i would be very thankful to you.