I can't play proper HD content through itunes via HDMI because my telly isn't proper 1080p HD or something - always comes up with a warning and then plays it full screen without sound. Works fine on the laptop screen with sound, the minute you unplug the HDMI cable.
Try going full screen on some SD content through iTunes...
is it trying to output the audio over hdmi when you're using it as a video only or something? (hold alt and click the sound icon on the finder bar thing to change outputs)
Is your TV 720p or 1080p?
I can't play proper HD content through itunes via HDMI because my telly isn't proper 1080p HD or something - always comes up with a warning and then plays it full screen without sound. Works fine on the laptop screen with sound, the minute you unplug the HDMI cable.
Try going full screen on some SD content through iTunes...