Here is an update: I was at my HO club today. A friend brought in a HO MTH PS3 SD70ACe diesel. It is a beautiful model. The club uses the NCE DCC system.
I tested the F6 sound feature. It worked. Pushing F6 allowed me to control the HO engine's sound volume from full off to full on and for multiple steps in-between. The owner finally settled on leaving the sound at about the 1/3 level. Even a HO engine can make a lot of noise.
Clearly F6 is how to control the sound volume on PS3 engines in the DCC mode. I just don't understand why the HO engine has better control than my O gauge engine.
NH Joe