There are car sounds that have been noticeably improved for sure, no doubt. However, just to give you some examples, these cars had far more depth and detail in their "audio character" in Forza 5:
* F12 Berlinetta
* F430 Scuderia
* SRT Dodge Viper
* 2002 M3 (or was it 2004? The one with the upgradable M3 GTR bumper...)
* 2013 M5 (the twin turbo one - sorry, I'm terrible at recalling year of manufacture)
* Nissan 240 SX
* Dodge Charger and Camaro (60s/70s)
* Z06 (the one from the V.Deisel movie XXX)
* ZR-1 supercharged
* ACR Dodge Viper
Many other examples - I'll have to revisit the game to come up with a more comprehensive list.
In short, I find some of the off-throttle and downshifting sounds unrealistic, in some cases, even cartoony.
Forza 5 - It's due to the higher dynamic frequency range and detail level/sample quality that you're hearing louder sounds - the AI cars around you are just as loud, especially when you go off-throttle. Check out AI cars passing by the camera during replays, you can hear their engines so distinctly and clearly. Load up a F6 replay, no sirree...same depth, detail and distinction is missing.
Mate, are you on TV speakers or home theater?