Thanks everyone for trying so hard to help!
I got snap to work in the track view, but I still can't get it to work in the PRV.
I un-checked "Snap to Nearest Audio Zero Crossings", and snap worked correctly in the track view.
Still, in PRV, despite all my experimenting, the now marker goes wherever I click, instead of to the nearest snap position.
Is there a separate setting for PRV snap? I can only find one snap preferences window: Preferences -> Customization -> Snap to Grid.
Thanks,
Mike