Ok Folks, now for resolution: Harry tested the engine on his layout and the sound worked intermittently. He took the tender in the back, came back and placed it back on the track. Worked perfectly. The answer: DIRTY WHEELS.
I forgot to check that. In HO that was the most frequent problem. In O it is rarely a problem, but because S is 2 rain, like HO - it pays to check the wheels frequently.
Thanks to Harry and all the folks at Hennings!