Well, I'm hoping this will fall into the "new game engine" thing, that PD will be forced to re-create GT from scratch because of the multi-threaded architecture, rather than simply porting and tweaking the same old game engine like they've been doing since GT1.
We know PS3 will be able to do better sound. Ungodly numbers of audio channels, as well as full-bore Dolby Digital/DTS 5.1 surround, which is a damn sight better than the Pro Logic II that PS2 is limited to (PLII ain't bad, but it's no match for discrete audio).
I was thinking about this the other day, and I think I have a theory that may explain some of it. I think a lot of it may boil down to storage space. Remember, they actually have to store sound samples, in addition to programming that tells the system how those samples work together. Uncompressed audio is big.. The sound samples for one car could easily take over 1MB, per car. That's over 700MB in storage on the disc, just for the engine sounds. That's probably unacceptable.. not just for real estate on the disc, but also load times and system memory. So, they have to compress it. Which means limiting the bitrate. And then a bit more, and then a bit more... until you have a sound that, while "taken from the original recordings of the real cars", is now just a synthetic-sounding shadow of what it used to be. And that's what we ended up with. This would explain why the demos had better sound than the final version of the game.. because the demos only had audio for a half a dozen different cars, rather than seven hundred of them.
Now, however, we have Blu-Ray. Need 1GB for audio? Have at it, there's 49GB more we can use for everything else. Of all the games out there, I think GT is one of the more likely to be using Blu-Ray out of the gate (remember that GT4 is one of just a few games that uses a double-layer DVD).
Combine the freedom of storage space, memory, power, a brand-spanking-new game engine, all-new audio tools and full Dolby 5.1? Oh, yeah... the audio will be a LOT better.
And if it's not, then KY is going to have some explaining to do. You know people will ask that if the worst happens... "Why does GT5, the greatest racing game on the planet, have the absolute worst sound effects?" And he'll be stumped for an answer, I'm sure.
For those of you who think the sound in GT4 isn't that bad, go watch a video of the McLaren-Mercedes SLR in real-life, then go play it in the game. That car actually pisses me off... it's one of the coolest cars in the game, but it sounds like absolute crap. I can't even stand to drive it. And that's what pisses me off, because I WANT to drive it.