Rasp. I havent found anything to complain about, and I wouldnt call this a complaint, but if the sounds could get some brutalness to them it would take em that little step closer what it sounds like when you are next/in a race car. Those exhaust explosions and exhaust backfires with their rasping sounds are just pure music. Also little bass level increase wouldnt hurt. I have a good headset that I use so the ones who disagree, id like to make that clear that its not about my system.
Honestly, the engine sounds should be generated in an entirely new way. Sims are all using the same pitch shifting of multiple wav files and it almost never sounds real. The way I think they should do it is make a sound print (defined as a text file of Frequency, Gain and Reverb) and "build" each cylinder fire sound. Perhaps an exhaust/muffler/resonator definition that then modifies the raw exhaust note. Smooths, focuses and defines the output direction and gain. An interesting thought. Some real life studies would need to be done. Firing one of an X volume cylinder and recording the resulting output through different size and shaped valve openings. Then firing that through a series of exhaust layouts and lengths. Get a baseline for "faking" it (8000rpm, V8, 4 stroke...) 533 Explosions per second. One hell of a simulation requirement but THE HELL LETS DO THIS RIGHT!
If they just used the Richard Burns Rally, Live for Speed, and I think old Netkar Pro (only up until V1.03) way, but massively improved on it, it would be wayyyy better. But they would need to work on it massively. The sound is sort of tied in with engine physics stuff, intake, exhaust stuff. Not really sure, I like it the best in Richard Burns Rally. In replay mode it really gives off that "I can tell thats a loud aggressive sounding car, without having to crank my speakers up" type of sound, just like real life videos do. Where as RFactor 2 and almost all other racing games, you have to crank your speakers up. I have seen some articles in the past regarding the sound engine in Richard Burns Rally, and its motor physics as well. Very cool stuff.
The sound mods for RBR are quite extensive and the ability to adjust multitude of different sounds in very precise manner is a wonderful thing. As you mentioned, that brings out the "aggressivenes" of the vehicles and makes the sounds very flamboyant. I find the sounds being in the meter of importance when it comes to realism and simlikeness up there with, and even above, the graphics right after handling/feel and physics.
