Right click the volume icon in the taskbar, select playback devices. Right click in the empty space and select show disabled/disconnected devices.
If the AMD device doesn't show up, you don't have the driver installed. If it shows up as disabled, enable and use it. If it shows up as disconnected, then there is something wrong with the HDMI connection (like using a non-audio HDMI port on the TV, damaged cable etc.).
CPU: AMD Phenom II X4 810 @ 3250MHz | RAM: Kingmax 2x2GB DDR2 800 @ 833MHz| MoBo: MSI K9A2 CF v1.0 (BIOS: 1.D)| GPU: Asus HD 6850 1024MB (DirectCu) @ 850/1150MHz | Display: L24FHD | PSU: PC Power & Cooling Silencer 750 Quad | OS: MS Windows 7 Pro x64
CPU: Pentium 4 Northwood S478 @ 3200MHz | RAM: 1,5GB DDR 400| MoBo: Gigabyte GA-8S661FXMP-RZ | GPU: ASUS Radeon X1650 Pro 256MB | Display: Dimarson 19" CRT | PSU: Noname 400W | OS: changing twice every week...