This is undoubtedly too late, but...
You didn't mention if you had any online plugins installed, but the impression I get is that you're playing RBR as vanilla, in which case the following will probably help you (or indeed others with a similar problem) The first thing you should do is to download DATtool:
http://www.kegetys.fi/rbr/. Extract dattool.exe and whatever things that may come with the pack to RBR folder.
Next I will over-complicate things for a laugh, bear with me.
Now, you have 2 options: either you use the original audio.dat as the source of all things audio, OR you force the game to read the audio files from a separate audio folder. I recommend you the latter, so you would have audio.dat as a mere backup from then on. In the end, it's much easier to change audio files whenever you want without having to go through all the extracting and repacking every single time.
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Use audio.dat as the source (option 1):
However, if you wish to keep using the audio.dat as the source, still you should backup audio.dat by moving it to some place else, or just by renaming it. Either way will do. This is to avoid any conflicts when you do the repack. So, drag audio.dat to dattool.exe, and DATtool will extract the contents to a new folder called "audio".
The car sounds are all located in RBR\audio\C#\... ... ...\Audio\Cars\. Put the lancer_evo_X.eng file in there. Check that Cars.ini isn't a read-only file, and then edit the file with notepad (write "lancer_evo_X" to the same car slot you have Mitsubishi installed in the game). Save Cars.ini.
Then do the editing you want in the audio folder, and then repack by dragging the folder to dattool.exe, which results in a fresh audio.dat.
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How to force the game to read from audio folder (option 2):
Once you've extracted audio.dat with dattool.exe, go to folder "audio", and browse through until you get to folder "Audio" (note the big initial). CUT this folder, go all the way back to the main RBR folder, and PASTE. "You already have a folder called audio or whatever, would you still like to proceed or a phrase to that effect", oh please do.
Now you should have the necessary folders and .ini files right there in \RBR\audio\. And you can delete the folder C# as useless. And to put the finishing touch, rename the original audio.dat, for instance:
audio.dat -> audio-original.dat
Now RBR will read the audio files from a folder and the car sounds are all located in RBR\audio\Cars\. Put the lancer_evo_X.eng file in there. Check that Cars.ini isn't a read-only file, and then edit the file with notepad (write "lancer_evo_X" to the same car slot you have Mitsubishi installed in the game). Save Cars.ini.
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This is basically it. Piece of piss, if I may say so and even if I may not. Of course, I do acknowledge that the instructions may not work or will bore you to death, but let's make it clear: I will not take any responsibility of anything bad that may happen during the reading of these instructions.
