There should be a button on either your keyboard or in the bottom right of your desktop that allows you to lower your volume.
on a more serious note; in gmod set the game sounds lower but leave voice high. but the car sounds are just loud
As a more serious reply, it's possible to make them quieter (through code) just means I have to go through nearly a billion sound definitions and change it myself. So it will take some time doing that, but I do understand fully what you mean about car sounds generally being far too loud. Don't be surprised if suddenly your car is quieter one of these days.