Sound scratchy if in a surround sound mode - OK...

Hi,

The receiver had been working fine until yesterday. Now when I select a mode like the 5 channel stereo or "matrix" it sounds really scratchy from all speakers. It sounds fine if I switch back to stereo or direct (but the sound only then comes out from the front left and right speakers). I disconnected and reconnected all the speakers. This is true for playing even the radio. Is the receiver busted and can it be fixed? Did we do anything to cause this problem?
Thanks for your help,
Ron