Microphone doesn't work on Gigayte GA-8I915P

Try removing the CMOS battery and wait 2 minutes, then replace.
Check the voltage before re-installing it.
Also, check and reseat the memory and processor.
While you have the board out, you should check all the USB ports for pin shortage.
Hope this helps.

worst scenario is that your Mboard is probably damaged or your board and your CPU is incompatible, my best sulotion is to get a new board for your CPU,.

http://www.gigabyte.com/products/product-page.aspx?pid=2255&dl=1#sp
According to their link, it will only support 2GB ram. 1 gb per slot max.

This motherboard can take a DDR2 RAM , this are the supported RAM for it Dual Channel DDRII 533 / 400 or Dual Channel DDR 400 / 333.

I can see why its so hard to look for it. their site has been changed dramatically. Here is the link to the driver for your motherboard.

http://www.giga-byte.com/products/product-page.aspx?pid=1815&dl=1#dl

You can download Gigabyte Mother driver from this Site. GIgabyte Mother Board Drivers

go to volume control option and select mic advace option and choose microphone boost option and try.

Maybe it's not recognized. Unplug every audio device, open Realtek HD Audio Manager and start plugging them back one at a time, the manager should automatically detect that something was plugged in a specific port. Just tell it what you have plugged.

You can download the manual here

I went to dogpile.com typed in "gigabyte ga-7dx" for the search, looked for the gigayte web site, clicked on the link and the page popped up. I selected to download the manual and can now read it. Just follow these steps and i'm sure you will have the manual too. It is all there. Best of luck.