I have a few things I'd like from you. The first being a copy of your dxdiag (Start -> Run -> dxdiag -> Save All Information). Just copy & paste it here, or upload it elsewhere and provide a link - and I'll take a look at it.
Secondly, what playback devices are enabled in your Sound Panel when you are connected to the HDMI display? You can try disabling all other playback devices except for the one you want to listen to, and then set the remaining device as the 'Primary'.
Let me know how it goes,