You have ALC888. Is the AppleHDA you are using from Lion? If so you will have to patch AppleHDA for HDMI audio with your card. If you use a ML appleHDA it will work OOB with the edit you have.
HDDMI audio is loaded in your system.
Thanks for your prompt reply.
Yes I have patched several AppleHDA.kext but all are not working for HDMI audio.
I mean I can find HDMI audio in "About This Mac"(System Information), but there is no such a option of audio output in System Preference.
In the past I have successfully enabled ATI HD 5770, 5850 & 6870 as well as Intel HD 4000 with extra GTX 680 in 5 different PCs.
This is the first time I can not make it working.
According to Tolera's theory this GTX 550Ti seems never working properly for its HDMI audio.
So I just ask your expert's opinion again.
Do you mean just use native ML AppleHDA.kext instead of patched one will solve my problem ?