2006 MacBook Pro 15" with DVI port - got $3 DVI to HDMI Adaptor which originally worked, then upgraded to Lion - now at 10.7.5 - and lost audio after one of the updates (can't remember which one).
Tried SMC and PRAM resets - no help.
HDMI used to show up as an output option in Sound panel - no longer.
HDMI does not appear as an option in Audio MIDI Setup utility.
Difficult to find audio pass-through option among DVI to HDMI adaptors for sale - some higher-priced options ~$80 found which really just separate out audio to separate jack for which an additional cable is then needed to bring audio to TV.
Frustrating to realize this was once an option which became deactivated by something Apple did without explanation or warning. It is becoming more frequent that updates take away something that simply worked and no effort is made to restore the lost functionality. Worse, additional purchases are made necessary to restore the function. Poor business ethics at best.