Microsoft has confirmed it's aware of the sound glitch occurring with HDMI connections to the New Xbox Experience.
First, turn off the Xbox 360 and remove the hard drive. Next, switch the console on and wait for the Dashboard to load. Once that's done, replace the hard drive, and hey presto, the sound should return.
Unfortunately, however, you have to do the same fix every time you turn your machine on, apparently.
The New Xbox Experience launched around the world last Wednesday, and most notably introduced Mii-like avatars to the console. And despite these sound complaints, the service has had a smooth roll-out.
Head over to our New Xbox Experience gamepage for our thoughts on the new Dashboard.