Encontrei este tópico com algumas pessoas com o mesmo problema que você descreveu.
I recently bought an HD 4670 with HDMI out for my HTPC setup. I was having the same issue as you, but after much trial and error, finally found a fix.
Uninstall the Realtek auido driver.
Restart your PC.
Allow windows to auto-install the basic Windows HDMI driver.
Now download the following ATI Audio driver:
http://support.amd.com/us/gpudownload/windows/pages/hdmi_xp.aspx?⟨=English
Download the ATI HDMI Audio Driver 9.2. Don't worry about the Microsoft one since Vista will have already done that for you.
I know that it says that it's for XP, but it works great in Vista. Install it, restart again.
Now go into the Sound Manger in Control Panel. Under the playback tab right click on the HDMI device, and select "Configure Speakers"
From there you should see the option for 5.1 sourround sound.