My NAD t 743 is a little over a year old. Out of the blue, it will only produce sound intermittently with a DVD or CD and will not run the TV audio at all. When it does produce sound with the DVD or CD it is only on certain sound modes and sometimes on the "a" speakers, sometimes on the "b" speakers, and, of course, sometimes on neither. Does anyone have any ideas why this is happening and what can be done to fix this? Thanks in advance.