No sound on SD channels only?

All,

Strange problem crept up just this past week.   I can get sound only in one situation:  optical cable, HD channel and U-Verse set to "Surround Sound."

I noticed it because all of a sudden I wasn't getting sound on SD channels.  Tried a variety of things:  changed to RCA cables (no sound at all in any mode,) changed to "Stereo" mode (no sound at all), tried various inputs into the stereo (same thing.)

Sounds like the U-Verse box deciding to put out 5.1 (and 5.1 only?)  

Any assistance would be much appreciated...  Thanks!

Adam