I understand what the writer is trying to convey, and have experienced it myself. He is somewhat overstating the effect, but on very high quality locks there is a different sound, AND feel as the lock is cycled through half and full cock positions. Saying it has musical qualities might be a bit of an embellishment, but I guess you could say there is a slight harmonic "ping" sound to it. And as I stated, the "feel" of the lock is just as impressive.
As far as the spring the writer describing, I think he is simply describing the trigger return spring found on most quality guns.
