On my Dell desktop( using HD 5700 )HDMI sound turns off after screensaver is active. HDMI sound works otherwise. It appears to be bug with 12.04, it was fine with previous Ubuntu releases.
This is not a problem with Ubuntu, but an issue with the ATI graphics driver. It happens when the driver detects the monitor going into standby, but does not detect the return.
Others have suggested running a beta version 12.7 of the driver.
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