not beeing familiar with Boeing stuff too much, I'm just making a guess.
Here are the little bits of what I have memorized on Boeing / MD
. Please, do not quote me on that, and if there are any eyperts out there who know better, I'd be glad to be educated.
, here's my guess:
DC-9 use afaik a drooped hinge-type High lift surface support and guidance mechanism. Meaning the flap surfaces are attached to a pivot arm. And that's why they seem to drop immediately.
Boeing 707-220 uses afaik arc tracks to deploy the flaps, whereas I do not know exactly how these are driven (Pinion/Rotary Actuator/Linear Actuator/... Anyone?) These will -most likely- move the flaps differently than a drooped hinge system does, the circular path they move along is not dependant to the length of a pivot arm (=Circle radius). Such an arc track could almost have "any" shape or radius. Hence, the movement of the flaps would look different.´Sound may be different depending on what kind of power drive is used.
Boeing 727 for instance uses a track-carriage type surface guidance and support mechanism. As well, Flap surfaces do not move on a circular path. The move somewhat more linear. Again, sound depends on type of power drive.
What the 717 does, I have no clue.
Anyhow, I'd say it is more a matter of the mechanical part of the flap system architecture and design that decides about the way the flap surface movement looks to a spectator, and as well it depends on the design and architecture of the power drive system what it sounds like. I would not say it is necessarily only a matter of aircraft age (=technology level). A hydraulic piston-actuated system will always sound different from a track-carriage system using hydraulic motors.......