After last years NXE update for the Xbox 360 several users encountered HDMI audio problems. This problem seems to be fixed now.
Here's a quote from Major Nelson's Blog (Official Xbox Live Blog)
"Starting Tuesday morning at 0200 Pacific when you sign into Xbox Live you’ll be prompted for a mandatory dashboard update. The update addresses the issue that a few folks were having with audio and their HDMI connection.
This update will be rolled out to all Xbox LIVE members around the world over the space of a few hours, so you may not get prompted for the update immediately. Thanks for your patience."
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