According to a http://www.avforums.com/forums/xbox-360/867084-nxe-installed-now-no-sound.html at the AVForums, several Xbox 360 users are losing all sound after upgrading to the New Xbox Experience.
Evidently 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 of the Xbox, you will have to receive the process. More PlayStation 3 News...