I lowered the volume of the effects by half and a bit the tires. I'm glad I can actually hear the other cars passing me, it was something that I really missed before. But I can hear my engine clearly too.
Master volume: 100%
Onboard volume: 100% (I've changed it from the default 25% but I'm not sure what's changed.)
Sound effects volume: 50%
Engine volume: 100%
Tire volume: 90%
I hope it becomes automatic and it just sounds like it should depending on the track, car and conditions. I'm not sure I've achieved that since I haven't ever been in a race car.