It has also been brought to my attention by a friend of mine, that following the installation of the NXE, some users have lost sound altogether. This problem is exclusive to users who have their sound running through a HDMI cable but strangely, not all are affected. My friend initially didnt encounter any issues during the update process but after switching his console on the following day, he found that he didnt have any audio output.
After a bit of research on the interwebs and after sifting through all of the temporary fixes (one of which recommended plugging in the hard drive with the console turned on!!!) I found a post on one of IGN's forums that seemed like a likely scenario.
According to the poster who phoned the My Xbox number, they were told that the NXE only utilised the AV portion of the cable meaning that the audio portion wasn't being used. A slight oversight for a console thats geared towards high-definition gaming in both visuals and sound. Presumably Microsoft will be releasing a patch in the near future.
In the meantime, those people who are experiencing a loss of sound will have to resort to using their component cable that shipped with their Xbox.