I for one do not like the GT6 sounds in the big picture as well.
There are a few exceptions, where sounds are not bad.
One of those exceptions for example is the Audi R18 winning Le Mans racer - although I remember the car to be a lot quieter than it is rendered in GT6.
It also makes a big difference, to race the R18 with a 5.1 system and set the sound output to "small theatre" + tweaking the position of the satellites and sound levels.
You can get the sound just right, that you get the direction and sound levels in a nice way.
For me the biggest point of critique is that somehow the car do not sound "alive" or "organic" or "real" when interacting with the car on the track.
It simply sounds as if one standard short sound sample for each car has been recorded and is played as a background sound, added overly dominant tire squeal and that is it.
In other simulators you can hear pop off valves, turbos spooling and singing, backfires growling, brake disks squealing and their squeal actually changing the tune as of how intense and long you are breaking.
You can hear the suspension, other kit in the car, as it knocks against the chassis, hear the chassis squeaking in intense track sections, …
For this kind of "alive" sounds you probably need as much hardware resources and programers, coding the sound engine, as you do for the graphics.
One single sound sample, played at a fixed rate, depending on very limited variables will not cut it either - we need a lot more different sound layers, that fluctuate, depending on conditions.
I always give PD the doubt, that the ageing PS3 system simply cannot provide those resources.
I see, how polished the graphics of the cars look now on the old PS3 and always think, that this is PD's primary driving force - looks.
I always liked such really bad reviewed games by comparison, as Shift 2 - although they severely lack in many points, one thing they do get somehow right is a feel of immersion, when at speed - partly of their exaggerated sound.
For me a really good example of total immersion (helped by fantastic sound) is and always will be Richard Burns Rally for PC. You can literally feel when things start to get dangerous, as there are simply so many cues apart from the visuals - the sounds, although on a very old software was always first rate.