Don't know what OS you have, but try:
- do not install to any "Program Files" folder
- run as admin
- use Win7, or even XP, compatibility mode
You can edit the files manually, but you will be in and out of the sim continuously to listen to the mix. Most of the editing is in the "// ENGINE REV-SAMPLE MIX" section. A typical subsection is:
EngineRPMPowerInside=1
{
MinimumRPM=1080
MaximumRPM=3046
NaturalRPM=2250
}
This example will play the interior power on sound designated by "VS_INSIDE_POWER_ENGINE_1=".
The wav will start playing at 1080rpm and stop at 2250 rpm. NaturalRPM designates where that wav will be played unaltered, below that the game sound engine will lower the pitch/tempo, above that it will raise it. The MinRpm must slightly overlap the previous sound MaxRPM, and the MaxRPM must slightly overlap the next sound MinRPM. Natural should always be between Min and Max. If you raise or lower Min or Max you usually must also adjust Natural porportionately to keep the pitch change constant (trust your ear here). I usually start with the power on idle sound and work my way up, hoping for a smooth blend all the way to just beyond redline (sometimes the wav files just don't want to cooperate), and do the reverse for the power off sounds, starting at redline work back to idle.
There are four sets of sounds to adjust - power on inside, power off inside, power on outside, power off outside; and each typically has 3-6 wav files to deal with.