I have a sony STR-K900 which came in a theatre in a box package. I have it connected to my PS3 with an optical cable for audio. When watching movies the dolby digital surround sound has always worked great (for about a year). All of a sudden, I can only hear background sound and when people talk it is either mumbles or mute during movies.
I have played endlessly with the receiver and found the sound is ok in 2CH mode only. I am thinking the receiver has a problem or my optical cable has a problem. I am looking for some advice before I go out and buy a new receiver and find out that is not the problem.
The optical cable is digital in that it either works or it doesn't so if both ends are firmly plugged in we can rule that out. By your description it is the center channel not correctly operating for some reason. The first question is if there is any output from the speaker. If not, I would carefully check the connections at both ends (the wire could well be sheared off and still look like it is connected) and if that doesn't solve the problem, try connecting a known good speaker to the center channel wire to see if you have a signal there.