Before installing the AC97 software drivers, open System than click the Hardware tab than click Device manager. Scroll down to Sound, video & game controllers. Do you see a yellow ! point? If so than your drivers do need to be reinstalled. I could send you to Asus's site but they are a bit out dated so go here for updated drivers. The first one listed under Description will do just fine.
When you open the sound, video & game controller find AC97 & uninstall by right clicking on AC97 & uninstall. Only the AC97, leave the rest alone. Restart the PC than install the new drivers that you saved to your hard drive.
Check driver installed by opening controll panel than open Sound & Audio Device icon. Under Audio tab Realtek AC97 should be listed than click the Voice tab & the same should be listed.
Test both Audio & Mic functions. If either fail your intergrated controller could be defective. Lets hope everything works fine.
Asus's driver download site for your P4P800.
Fill in under "Please Select the Product".
Than click GO