For best user experience it would be better to handle this on the client instead of the server. If it’s handled on the client, the effect is guaranteed to be smooth and accurate instead of laggy (that is, if you script it to be those things, too, which is what the next things are for).
Next, you may want to tween the sound’s values so they doesn’t instantly jump from point to point (making it feel ‘laggy’). Your code looks like it should do this roughly due to it updating so quickly, however even if it was run on the client, the replication would still add some latency to the velocity value itself. To fix this, you could use TweenService (won’t work very well if the value is constantly updating quickly) or another form of tweening (lerping the value with a single RenderStepped connection or something) which is more complicated but works even if it is being spammed. Whenever the values change, they should now smoothly tween to their target value from their starting value so it feels smooth for the player.
Another thing is to get rid of all those if statements. The best way to do this that I can think of is just simply interpolating the volume and playback speed based on the velocity. It’s kind of what you are already doing, but expanding the concept even further so that you don’t need any if statements at all. You will lose some accuracy from doing this (unless you further play with your equation), but it’ll be smooth and consistent.
e.g. volume = 0.5 + math.clamp(speed / 1000, 0, 0.5)
This is pretty much the same as what you are doing for each value, aside from the clamp that in this case stops the volume going over 1.5 or below 0. Just keep it in mind that this is possible instead of doing different values for each one.
The final thing is kinda small, and it is just to use variables. You don’t need to write out all those big paths heaps of times, and doing so isn’t very efficient either. Instead:
local parent = script.Parent
local engine = parent.Engine
local speed = parent.Velocity.Magnitude
if speed < 10 then
local smallSpeed = speed / 100
engine.Playing = true -- see note #1 and #2
engine.PlaybackSpeed = smallSpeed + 0.1 -- see note #3
engine.Volume = smallSpeed + 0.5
elseif speed < 125 then
local smallSpeed = speed / 250
engine.PlaybackSpeed = smallSpeed + 0.2
engine.Volume = smallSpeed + 0.6
end
As you can see, this is much more readable. Things can be understood at a glance, and you don’t need to make duplicate calculations and indexes.
Note #1: Playing it when the velocity is low wouldn’t work if the vehicle suddenly jumped in speed. You may have done this to avoid spamming the playing property, but the driver could also drive slowly and there wouldn’t be any point. I think you want to instead separate this from the sound calculations and force it to only run once (when the vehicle starts moving, at any speed).
Note #2: Maybe you should use the sound:Play() method instead? Not sure what your specific implementation is, though, so you can ignore that if I’m wrong.
Note #3: Using another variable here, since the calculation of smallSpeed is being done twice in the same scope. In addition to this, you don’t need to wrap the full mathematical expression in brackets if it’s a simple var = x statement.
Anyway, hopefully this is helpful! 
