Try using another HDMI cable to determine if the cable is bad
If there are other audio devices connected to the PC, a hardware/driver conflict can prevent the desired HDMI audio device from working correctly. Check the device manager to determine if there are any issues / conflicts highlighted for any listed devices
Right click on My Computer
Select Manage
In the left navigation pane, select Device Manager
Look for any notifications listed, usually marked by a yellow triangle with an exclamation mark
Right click on any item that is marked, select Properties
In the General tab, select the option to Troubleshoot
Follow the troubleshooting steps to determine how to resolve the issue / conflict
The motherboard’s BIOS settings may need to be configured to allow for the desired audio device work correctly
Refer to the motherboards manual or contact the manufacturer to determine the proper configuration for audio BIOS settings
A defective graphics card can prevent the desired HDMI audio device from working correctly
Try to connect another HDMI capable device to the HDTV using the same cable and input to determine if the audio works
If the audio works with an alternate HDMI device, the graphics card in the computer may be at fault. Contact the place of purchase to obtain warranty on the graphics card
source :
http://support.amd.com/us/kbarticles/Pages/GPU70NoAudiofromHDTV.aspx