I can't believe that people insist that the "No!" was intentionally made to stand out. Mistakes happen. For instance, Lucas dished out millions to work on A New Hope twice (original and special edition) and in both versions the problem of old Obi-Wan's fizzled-out looking lightsaber remains in his duel with Vader. The book states that Qui-Gon spoke the whole statement, and theories proposing that it was some one else's voice are too convoluted in this instance.