There have definitely been sweeping changes to the sound design.
Exhaust sounds are more distorted, except racing exhausts. There is also additional reverb. But the mix is still quite compressed (dynamic range), so the overall effect is to create a wall of sound, rather than the impressive dynamics of the real thing, or indeed of the PS3 era games.
I noticed some new effects, wind noise and impacts on curbs sound different - I think there are new "sweeteners", e.g. something intermittent that sounds like keys jangling that I've not heard before. The sound of the shifter in the gate for manual cars, especially older cars, sounds louder now. In fact, maybe these effects, like the turbo sounds, are just mixed at a higher level / more prominently.
The interior of weight-reduced cars is a little comically overly reverberant now, sounds like you've got your head in a bucket. But this is on a PS4.
I don't like the new gear changes. I got used to lifting and it sounded pretty good. Now it's as sterile as it ever used to be, again.
