To the extent this helps anyone - I also had the same problem discussed in this thread (I have a Sony Vaio VGN-FZ21Z, with a sigmatel high defintion audio and Nvidia Ge FOrce 8600M GS), and unfortunately none of the solutions worked for me.
HOWEVER... I recently tried again and this time got the HDMI audio to work... what i did was uninstall IDT audio from my list of programmes, and then restart the computer ! naturally windows reinstalled a high definition
driver and this
driver allows my now to hear audio through my hdmi!!!
This was purely discovered by accident (as I was going to try to stop windows installing a generic
driver and try to install an updated IDT audio file - but as windows 7 was too quick for me to stop it - I stumbled accros this 'solution').
I hope this helps others with the same configuration... and Happy holidays
UPDATE 28/12/09 - With this new generic driver I now no longer can use the builtin microphone on my computer... very annoying - the webcam works (in MSn but not skype?) but the microphone doesnt work (in any programme) - any suggestion would be warmly welcomed!