Interesting problem has developed. It seems that a very short power outage (less than 3 seconds) has somehow screwed up the DSP of the 4520.
Let me explain:
When using any other Listening Mode other than "Stereo", it sounds like spoken dialogue has been mixed out of the center channel into the very low frequencies. The rest of the surround sound signal sounds normal. The only way to hear dialogue is to disable the various multi-channel listening modes by scrolling over to "Stereo" - effectively negating any surround signals.
When viewing the video material, the effect is weird - you can see the actors speaking but it sounds like they are talking to you underwater - very faint, garbled, low frequency voices.
I've tested this with various source material - HBO cable, Netflix, blu-rays/DVD's.
I tried switching the AVR off - unplugging the power cord for a minute - but nothing seems to correct this. Note that the exact same material that was viewed the evening before exhibits this problem.
Does anyone have an idea of what's going on here?