Without sound I'm a little off pace, about a second or so. Thing is with sound, you only hear stuff after its happened, so you react to it, instead of taking control of it and preventing it from happening at all. So I don't believe it makes a big difference to laptimes if you pratice without sound. Racing in a pack though, you would be at a slight disadvantage.
If you have ever played a PS2 game called Enthusia, that has an interesting display system that I would imagine is very useful when played without sound. It has A G force display that also shows how loaded each individual tyre is, and how close you are to their traction limit. Maybe one day such a feature will be integrated into LFS, or written by a 3rd party as an overlay app.