If I am reading this correctly, ATI Mobility Radeon HD4500 family does not support HD bitstreaming.
Integrated HD audio controller with up to 2 channel 48 kHz stereo or multi-channel (7.1) AC3 enabling a plug-and-play cable-less audio solution
You will get DD/DTS 5.1/6.1/7.1 via HDMI not Dolby TruHD or DTS-HD MA via HDMI.
Having HDMI does not automatically guarantee HD audio.
You need the Mobility HD4830/4860 family to get HD Audio
HDMI output support
Integrated HD audio controller with support for stereo and multi-channel (up to 7.1) audio formats, including AC-3, AAC, DTS, DTS-HD & Dolby True-HD, enabling a plug-and-play audio solution over HDMI5