Just take a look at how the new X cars sound, how the Alpine 110 (premium) sound (most new cars in GT6 have very accurate idle-low rpm sounds, etc), and you'll see they feel different. The X cars really use a different method in their sound generation, Griffith wrote about this a lot of times in this thread.
Those ARE different sounds, made in a different way. So they do care about sounds and they are trying.
It's Polyphony Digital after all... they are all about simulating stuff. There's probably one sound guy and a programmer right now trying to record/simulate/play the exact sound that an explosion in a cylinder does, the sound of the air intake sucking air into the combustion chamber, the sound of the valves, the turbocharger bearings, the influence of the exhaust material and size of the tubes does in the sound generation...
And one day, they will come with a "sound simulation" just like they did with weather and day to night cycles. Mt. Panorama feels like it's Australia as in Goodwood the mood is completely british. They just need to take all the data.
They will get there, eventually. Just wait. For now, what they did in the X2014 junior, expanded to all other cars will make me happy enough for the PS3.