I see this when the tuner is on an SD station and I watch a dvr'd event. When I come back (though the dvr menu - NOT just hitting the live tv button) I will have no audio on SD stations until I change the channel to an HD channel. I think this a handshake issue with the tv.
Thought this was about a singer who quit. (Joey Volume) Would make a great singers name.... ok back to the problem.. Every once in a great while I have this happen with my 612. For me changing to any other channel not even going to an HD channel fixes it. And it only happens when I am going through my audio receiver, not directly to the TV. I too am using HDMI and always thought that was where the problem originates from.