According to a long thread at the AVForums, several Xbox 360 users are losing all sound after upgrading to the New Xbox Experience. Apparently, the problem only appears in HDMI-connected systems. but there has been one report of someone having the same problem using a SCART connector. Fortunately, there seems to be a fix.
• Turn off the Xbox 360.
• Remove the hard drive.
• Turn on the Xbox 360.
• When the Dashboard loads, put the hard drive back.
Apparently, the fix is only temporary. If you turn off the Xbox, you will have to relive the process. [Xbox360 Fanboy]