I am having the same issue with my 722k. It's software v. L753RMKB-N. I have not done any 'systematic' testing but it only seems to occur when I enter the channel number w/the remote (not when using channel up, channel down, or selecting a channel from the guide list). I've seen it happen the most when switching to FoxNews (205). One time, when switching to 205, the audio did come up after about 10 seconds. To get the audio back, I have to reset the receiver.
I am using the digital optical cable to an amp/stereo. The stereo indicates the signal it's receiving and it shows DD5.1 but then when the sound goes away it indicates no signal. (If the signal is incompatible with the stereo's settings, it will indicate that on the stereo.. but in this situation, it's as though NOTHING is being sent to the stereo.)
Of course, the receiver would go stupid every once in a while and I would have to reset. (like everyone does) But I've had this same setup for years now and never had this problem on a daily basis. I've noticed this significantly for a few weeks.