I have had my Asus P5b for a long time now and i have had this problem before but i cant fix it in the same way i did last time. the mother board has a built in sound card and it was working with no problems. I recently installed a new graphics card (9500 GT). the graphics work but i lost my sound. at first it was completely dead, no sound at all, and i tried updating the sound driver software, but i was already at the newest version. the speakers and cords are good, i tried them on another computer. i also went to the Asus website and installed the newest audio driver from there. it's called SoundMax Audio. now the speakers only give out a static sound that seems to increase with the more activity i have on my computer (like opening websites or playing games). someone please help me out, I CANT TAKE ANY MORE OF THE HISSING!!!!
I had the ASUS p5w, similar board, and i had a similar problem but I simply installed the latest drivers and it worked again. You could try several things, maybe try uninstalling the sound card drivers and reinstall an older driver. You could also check if its your speakers or headphones that are broken. Otherwise if you really cant make it work you could buy a new sound card. They are relatively cheap.
Hope this helps.