Hello dwatson3,
I've just checked, and all of the 16' diapasons on the Salisbury have non-silent bottom C notes.
Check that MIDI is being received from your bottom C key, and look at the audio level meter on the "Audio, MIDI and Performance" control panel when playing it to verify that audio is being played from the virtual pipe (you might need to turn up the level trim to see it more clearly). Also check your audio routing if needed.