The HDMI Audio driver should come as a part of the latest Cat 9.4 and Cat 9.5 drivers, so I would recommend you to uninstall any other drivers and then reinstalling them over again.
You should be able to right click on the Audio Devices inthe system tray and ATI HDMI should be listed there, selct it and then click on Set Default.
I also have Vista 64 Bit but the HD 4870 1GB in Crossfire, and I've never had problems with Sound over HDMI.
The only issue I had was, when running FarCry 2 in DX10 sound over HDMI would not work, but that was resolved in the last Cat 9.5 drivers.
Hope you get it fixed.
Cheers