I think the PG sound design is overall a bit better in the way it mixes engine and exhaust audio - a primary example being how the pitch and tone changes the moment you go off-throttle. This is missing from most cars in FM5. And, the exhaust pops, crackles and burble is more audible in Hor 2 as well, though the reverb isn’t quite as impressive when you’re driving in a tunnel.
In FM5, the reverb is insanely good, the engines are louder and more detailed, but then again… I can’t help but wonder how the cars just sound that much better in Hor 2 even though they’re not as loud. My solution: set tire screech at 9, sound effects at 7 and engine sound at 10… crank the volume way up!
I bought some Razr Carcharias awhile back, and when I’m on FM4 driving something like an '00 Cobra R there’s actually a lot of burbling when you let off the throttle. Heck, you can ever hear belts and pulleys moving if you let most cars idle. But when I’m using TV speakers I can’t hear any of that, because the burbling is almost entirely bass, and the belts are really subtle. It helps to have a decent sound system while playing these games.
I initially had the game on just stereo speakers from an amp. I have one of those mini-component budget Sony surround systems that do surround sound ONLY through the built-in DVD/CD drive! But Since it does come with 5 speakers, I’m using 4 of them in total (minus the center surround) - 2 mid-range speakers, 2 satellites and 1 sub-woofer with the game set to “Theatre”. I later through in a pair of Creative satellites with another sub-woofer, which is actually fairly powerful for its size and price.
Even though, my set up is 2.1 digital stereo… the high and low frequency sounds coming from a total of 8 speakers has nearly transformed the experience… as you say… the exhaust burble…reverb effect… turbo whoosh/whistle… is clearly audible, along with feedback from the tires and those wonderful idle sounds.
I would honestly say, FM4 had a slightly better sound design… though FM5 has better mixing.
Sorry, I posted that boring mini-essay above… just felt like sharing my 2!