Jun 14, 2009
I built a computer and my case had AC 97 audio and my board was Intel HD I didn't know it was AC 97 and when I plugged everything in the headphones hissed Did I damage the motherboard
No, you didn't damage your motherboard. AC97 is compatible with HD audio however it doesn't have all the features. There are a couple of options you can use to upgrade your front audio to HD.

As the frontx site says "consult your motherboard manufacturer" Also a good idea to check your case manufacturer in case there's better/cheaper solution. Some Rosewill cases have a AC97 and a HD audio dongle