Audio problem: Spock uses the intercom to call Mr. Scott who is in engineering. As the shot cuts to a side view of Scotty standing at the controls, he answers "Scott here" but his mouth or lips never move. (00:44:24)
Audio problem: When Chekov goes berserk on the bridge, he emits a scream and shoves Spock. When Spock holds Chekov's face, the closeup on Chekov shows a scared look, and the original scream is played again, but Checkov's mouth doesn't match.
Chosen answer: If they're in orbit, they're being pulled along by the planet's gravity well, therefore, impulse engines would only be used for minor corrections and would be "on standby" while in orbit, but not active. (Like keeping your car idling without revving the engine and creating plumes of exhaust).
Captain Defenestrator
Thank you for the info.
Movie Nut