I never noticed that was missing, but WillLem is right.
I'm undecided about it.
On the one hand, it would be a way to increase similarity to original Lemmings again.
On the other hand, it seems like NeoLemmix is moving more and more away from original Lemmings even in its aesthetics now. And I agree with IchoTolo that this sound can be annoying. Because most likely, there wouldn't be a way to only turn off the release-rate sound while keeping all other sounds active?
Of course, you could hit "s" on your keyboard before changing the release rate to deactivate the sound, and then hit "s" again to turn it back on. But that's just additional inconvenience.
Also, what should happen when using the right-click shortcut to immediately set the release rate to either 99 or the starting minimum? Just a quick "blrrrrp"?