The corporate giant has confirmed there are indeed troubles for some users with NXE and audio output through HDMI, it's to do with a protocol update.
Microsoft says they're "diligently monitoring this issue" and working on a fix. The only workaround at the moment is to switch from HDMI to analog cables to get sound.
"While the vast majority of our users worldwide are enjoying the New Xbox Experience with no problems, we are aware that a handful of Xbox LIVE users are experiencing audio issues, and are diligently monitoring this issue and working towards a solution," read Microsoft's statement.
"We appreciate our members' enthusiasm and patience, and apologize for any inconveniences."
Another workaround that seems to fix the problem for some is to detach the hard drive and then reattach it once the dashboard has fully booted up, but must be repeated each time the console is powered up. Let's hope the Microsoft coders get this sorted real soon.