You do know it sounds like crap right? They dont sound anything like a real car. It has been talked about soooo many times.
Bottom line = GT5 sounds are terrible.
Pyramids on mars have been talked about sooo many times as well. That doesn't make it true.
The sounds are brilliant as usual. You wont hear the full benefit unless you have a top end budget surround sound system with at least optical sound. I have a mid range yamaha surround sound bundle with a sony amp with optical, but even that doesn't do the sound full justice.
Someone already said before that you need to watch out for what view you are using. That's correct. When in a car when a rear engine car and in the cockpit, yet will be hearing the engine and exhaust and gearbox all from your small rear surround speakers. You could fiddle with the speaker set up and make the rear speakers louder, but even at that, you have so much continuous sound data going through an ultimately "lossy" system.
Think of it as trying to listen to someone whisper in a movie, while in the background there is am explosion, heavy rain and waves crashing against rocks.
And if you're listening to it through normal tv speakers that's worse.
The third person view you will just hear mostly the exhaust. Which at high revs will sound like one continuous whine. You will hardly ever hear people in real life giving their car full bananas. So give in about half revs with a bit of volume on your hopefully at least mid range surround system for a reference.
Lastly, for those that have actually been in or near a sports or racing car being driven in anger, will know that you usually need some sort of ear protection for any prolonged time around the noise. Not recommended, but if you have a decent sound system, turn in up till you feel you need ear plugs then you're talking!