For me, the keys (enter option back a-z etc.) always have an audible click. For the silver nav bar, up/down also clicks, but left/right can be completely silent if you press softly enough.
in general does the sound affect the experience of using the ereader?
I sleep alone so I don't care... my PC keyboard is definitely a lot louder and I type a lot more on it... and wouldn't trade it for any other keyboard (cherry g80-3000 with click keys, so it's actually a feature).