There's quite a thread going at the AVForums in which several 360 owners are saying that the New Xbox Experience has caused their Xbox 360 consoles to lose all sound. The problem seems to be isolated to those who use HDMI to connect the console to their television sets, although one user has complained of the same problem using a SCART connection. For the moment, a quick fix has been discovered that seems to work for many people. If you are experiencing this glitch try the following:
While the Xbox 360 is turned off, remove the hard drive.
Turn on the Xbox 360.
Once the Dashboard loads, replace the hard drive.
Reportedly, this fix only lasts while the Xbox is on. Once you turn it off, you'll have to repeat the steps above to make the sound work again. We've contacted Microsoft and the company is aware of the issue. We'll update this post when we receive more information.