I have been thinking about these sound issues and I think that there should be possibility to adjust each sound separately, because if scawen sets sounds to certain level it is fine on his system and for his ears, but only there it would be as intended, different speakers and ears are certainly causing different experience and only if all levels are adjustable it is possible to create similar sound enviroment.
So there is need to get sliders at least for gear whine, gear click, bov, turbo spool. Engine, echo and skids are already in game, but I have no idea how hard adding these would be.