While it's true that Forza 6 has lost some of the rawness that FM4 and even FM3 had (only Forza games I've played and had), comparing them side to side, I have to say the sounds are much more detailed in FM6. While I loved FM4's sounds in particular, it was a bit odd having every car sounding like they had a race exhaust. In some cases a bit too exaggerated. This is what I think about Forza 6 sounds:
4 Cylinders: FM6 has much better 4 cylinder sounds for most cars. However, some others have gotten a bit bland and windy. Boxers are always fantastic and an example for GT to follow (even though GT gets the VW boxers better). Forza is still the only game that gets Hondas right but also mimics V-Tech (can't believe I mentioned this... ugh! Shame on me! But it's true). Making the most boring and common engine configuration (L4) sound right is a feat by itself and I have to say, most 4 bangers in FM6 are crisp and throaty, which I like! Some others disappointed me a bit like the Lancia Fulvia which is missing that characteristic boxer-ish sound from its unique V4 engine (more of a "VR4"). But others.. simply wow... My favorite has to be the Mercedes 190E Evo.
5 Cylinder: An engine that is pretty much straight forward. Or you get it right, or you must work for PD. T10 knows how 5 Cylinders should sound and they got them right. the sound is so characteristic that they could've copy-paste all of them, yet they were very careful not to make them all sound the same which is very appreciated. That Audi Quattro man... beautiful.
6 Cylinder: So far I haven't found a single 6 cylinder that I don't like. Maybe the Supra has changed a bit and it isn't as good as before. Also, the M4 isn't that great (just like in real life). The Fairlady Z33 and Z34 are a bit muffled as well, but they do have their characteristic Nissan V6 grunt. The GT-R is quite fantastic and as real as it gets; especially at low revs. The Fairlady Z32 is simply eargasmic. BMW L6's are beautifully done as it should. A very pleasant surprise was the pushrod V6 from the From the Ford Capri. That's how an old V6 should sound like!
8 Cylinder: a lot more variety when it comes to V8's. This is a very pleasing surprise as a V8 unless it's a flatplane or a BMW TT V8, the sound (as with 5 cylinders) is very straight forward. They could've easily copy-paste most of them as I mentioned before, but they didn't (except for those cars that have the same exact engine). I do agree, the Jaguar V8's are a bit lacking and it almost sounds (forgive me) as the best V8 Gran Turismo can do. Don't know what happened there. Muscle cars are different animal. I could go hours playing around on then at the test track. and oh boy... the Mercedes 300SLR!! That L8... bravo, Turn 10.
10 Cylinder: These are simply exquisite and there is a huge improvement in the Vipers for example (especially the old GTS). The new Viper and the last ACR are definitely improved. The LFA is indeed lacking a bit of its spark, but it's much better than in previous games. The R8 is probably the best, hands down. Followed close by the RS6 which it even has its very characteristic almost V8-ish grunt at mid-low revs just like in real life.
12 Cylinder: There are probably just a few that really manage to melt my ears in ecstasy like the Diablo, which didn't sound too good in previous games. The F50 was unbareable in previous Forza games due to the awful sound and they've toned down that annoying screaming sound. The Veneno is quite spot-on, although the Aventador is missing something. The Zonda Cinque was a bit of a disappointment; PCars gets it better. Huayra on the other hand is absolutely gorgeous; it's pretty silent, "windy" and almost boring, but that's how the car sounds in real life. Aston's are indeed lacking a bit as well, but they are still great sounding cars. All in all, at least they all sound like V12's.
16 Cylinder: Ah the Veyron... Not really my cup of tea. However, I can understand why they gave it a V8 sample, as this is the basis of any 16 cylinder song, but I feel like they made it too V8-ish just like in previous Forza's. I'll stay with the EB110. I was always a better car anyway.
Rotaries: Are just... bad. Sorry. No Forza game has made them enjoyable. While the sound is definitely of a Rotary engine and now they even got the "brap, brap, brap" at idle, the rest is just absolutely obnoxious! The 787 sounds beautiful but it's missing something that even GT got closer to the real thing. However, GT had awful bland, artificial sounds, so it didn't help the good sample.
At the end is just a matter of opinion. Personally, I prefer above all, for the samples to be correct and match the engine types. Second, that cars which a characteristic sound are given priority. Third, that they don't over-do street cars or sounds as a whole (a.k.a distorted or too loud). And last comes the details. Forza has that organic feel to their sounds, especially in this game that sets it apart. You can hear so many layers and variations in a single engine depending on revs and how hard you're hitting the throttle that to me it is the closest sound simulation in any game that's I've heard so far (and one of the top three aspects of why I have turned to Forza).
And no, it's not about the sound system (although it helps a bit), it's about the composition and execution of the sound itself. If the sound is done right, it will not matter if you're using headphones, tv speakers or a $10,000 sound system. PCars comes close, but there is something odd about driving a street car that sounds like it's completely stripped down, with a straight-gear tranny, and open pipes (then again, the purpose of Pcars is more about the racing).
And also... remember I'm an OCD car enthusiast and connoisseur that came to Forza after more than 15 years of being an angry and frustrated Gran Turismo fanboy, so... yeah XD