Quoting myself here...
This isn't that hard actually. You do have to get the mix correct however. What they've done in fmod basically (page 188 of the manual http://www.mediafire.com/file/tol52mvp24bzpfg/fmod_designer.pdf/file ) is something like this. So if you extract the sounds and remix/replace them using the same lengths and sounds that blend together nicely it's do-able. If you're not familiar with fmod what's basically happening is that as you increase rpm/load/speed the sound played is "load" and the "unload" sound is blended out.
When the fsb is extracted you'll have a bunch of rpm files, sometimes they're named load/unload, sometimes in/load, the "outs" are the external engine sounds. What you'll notice when playing them is that the load sounds have a lot of "wind" sound mixed into the .wav file. What I've done in http://www.mediafire.com/file/g3t9or9urp3mrs5/engines_mix_3006_beta.rar/file is mix the external engine sounds (using audacity) with the "load" sounds, making the original "wind" mix less noticeable and allowing the engine sounds to come through more.
To use fsb extract (https://github.com/gdawg/fsbext) put it in your "working folder" and create a couple shortcuts to the fsbext.exe naming one "extract" and the other "rebuild". Add the following to the "extract" shortcut.
Add this to your "rebuild shortcut.
Create a folder called named "myfolder" in your fsbextract folder, copy main.fsb to your fsbextract folder, double click the extract shortcut, edit the sound files (don't delete files, the rebuild will error if it doesn't find the same names, use a blank wav) , double click the "rebuild" shortcut and it'll create the file output.fsb. Output.fsb is loadable in game after renaming to main.fsb.
If you're starting over with a "fresh" main.fsb delete files.dat.