voodooHDA driver can make my audio card on optiplex 755 work, however, there exists some static background noise in the headphone.
I have tried several settings both in the sound preference of leopard and the voodoo preference panel:
Go to system preference ---> sound ---> input, adjust the input volume to be zero
Go to system preference ---> others ---> voodoo: select line out, and adjust the input gain to be zero, then select microphone, adjust input gain, speaker, and mic to be zeros.
Now, I got a perfect working hackintosh machine, with audio, video, network, and all stuffs working.