You have to track it before you do the .Changed:
local SliderHolder = InspectCutscenePartFrame.PartSettingsFrame.DurationSettingsFrame.SliderFrameHolder
local FOVSlider = Slider.new(SliderHolder, {
SliderData = {Start = 1, End = 50, Increment = 1},
MoveInfo = TweenInfo.new(0.1, Enum.EasingStyle.Quad)
})
FOVSlider:Track()
FOVSlider.Changed:Connect(function()
print("Changed..")
end)