I think the engines currently sound a little tame currently, considering their power output and the weight of the vehicles. Not to mention the agressive/exciting nature of the game. I like the music, but I think you should be able to turn off music on a racing game and still have exciting sound.
The after-burner sound of the boost is cool though, as is the gear-change 'pop'.
I'd like more bass or roar to it, rather than the current whine (they sound like Wipeout-style anti-gravity drives).
It's hard to find anything with enough bass that still sounds fast (as opposed to a Harley or truck). Most supercars scream which wouldn't be right for vehicles weighing tons. The closest I can find quickly is this: https://www.youtube.com/watch?v=Gppp-U4jgqI