So a couple issues with the car sounds on FH5, the first of which applies to most cars really but is more noticeable on some classic muscle cars and a few Honda vtecs where the engine is not revving as high as the tachometer on the HUD makes out, so for example on some of the classic muscle cars the HUD tachometer is reading 6k rpm whereas the engine sound to a more trained ear is more like 4k rpm, there needs to be a more far more accurate synchronisation between the rpm reading and the actual engine sound, which hopefully would be a case of a bit of sound file editing?
The other main issue is that the gear whine when selecting an upgraded transmission or even a drivetrain swap is borderline ridiculous and is capable of causing headaches so ideally it should be removed or the option should be there to turn it down/off completely.