The moon's uneven gravity field presents a challenge to ground controllers planning trajectories for low-altitude lunar orbiters. The tug of lunar gravity can alter a satellite's orbit, requiring frequent rocket burns to adjust the spacecraft's path around the moon.
How did you manage to have a "plain old soundcard" that could put out 60kHz? Since human hearing tops out at 20kHz, standard sound cards have a maximum sampling rate of 48kHz, so there's no way you could get more than 24kHz out of that. Perhaps whatever aliased frequency did come out happened to produce enough resonance at 60kHz to satisfy the receiver?
The whole purpose of this mission was to measure the variations in the moon's gravity by flying at a very low altitude. Consequently, those gravitational variations introduced changes in the orbit requiring relatively frequent corrections. It follows that the closer you orbit, the more actively you'll have to work to maintain that orbit. When you fly low over a mountain, the extra mass in that area will pull you down, and you'll have to correct for that with upward thrust.
That would be why it was linked in the summary.