So I went to watch the replay thinking maybe I'll hear a trace of the old sounds but surprisingly One interesting thing about the Red Bull Sounds is that unlike the BMW V12 LMR, there is no trace of the sound in the replay or in the Data logger. They seem completely separate. I can't hear any trace of the Old sounds from GT5 in the replay and likewise, no trace of the GT6 sounds in the Data logger. I don't how that's even possible, but its really like night & day between the two.
Someone complained, almost immediately on its release, that the sound of the FT-1 in the Data Logger was "too smooth", so naturally I gave it a go myself. It uses filtered control data from the replay (so much less than the physics rate you get in-race) which causes a lot of the weirdness, and there is no localisation (spatial mixing) and extra effects.
But then I had the idea to try the Red Bull cars, knowing what I knew about the sounds they made; sure enough, the new components of the sound were missing, to be "replaced" by some awful-sounding samples. So far, so similar, but I also thought that these samples can't be heard in the game normally, but I was wrong (someone else had already told me that they thought there was some other sound, though).
Slightly more recently, and at the risk of deafening myself, I turned the volume right up with headphones after hearing some strange noises in the middle of the rev range of the Junior. Using H-pattern, you can get the engine to tick-over slower than 2k rpm, and sure enough, that's where the samples are most audible. The actual exhaust sound that makes all the noise is definitely synthetic, and it cuts out around 600 rpm - at that speed, it sounds awfully similar to some of the things I've tried (sort of "puffy", steam-engine-like), and there is no hint of the crunchiness you get with trying to pitch shift that slowly.
The samples, though, add a bit of texture around the idle speed. The problem with idle instabilities in a real engine is that it's a dynamic part of how the engine runs, and is caused by interactions of various acoustic, inertial and control aspects, distributed throughout the engine, that it's very difficult to actually "simulate". So an idle loop at the bottom end can fill that in.
However, the other samples are still there as well; as you progress through the rev range, you can hear them pitch shifting, clashing against the exhaust sound. It makes the samples sound even worse (and is a nice demonstration for those who might otherwise think pitch shifting is fine; I can't stand it now I've heard the alternatives.)
So the Red Bull cars have the new exhaust sound and traditional samples "playing" at the same time. Quite interesting.
I've not heard the V12 LMR, but I expect the difference is largely attributable to the exhaust flow noise effect that was introduced with GT5 Prologue (and the spatialisation I already mentioned). The BMW VGT suffers from that particularly, as it has twin exhausts; so the same sampled sound is played from each exhaust and the flow noise (modulated by the sound coming from the exhaust, i.e. the samples) is also duplicated at each location. So you get double flow noise (independent sounds / sources per pipe would not "double up" so much, they'd interleave better), made worse by the fact that the samples already have flow noise in them, and you also get the comb filter effect from the doubled sample sound.
This is clearly a provision for the new sound method; each pipe can easily get its own sound and the flow noise won't be so obnoxious. They could potentially, partially, "fix" the issue by delaying one pipe slightly, but it really needs a totally independent sound. They could turn off the flow noise, but you'd lose a large portion of the spatial character, making it sound dull in a different way.