How i think GT5P Spec III sounds:
Tires: Okay, the overall noise is still too loud on racing tires and it doesnt change often enough, but its comprehensible(?).
Engines: Characteristics of each car are spot on, a Ferrari sounds "Ferrarish" a Corvette "Corvetteish" and Lancia Delta "Lancia Deltaish".
But every car sounds flat, dead and thin. Theres no impact in any of the engine sounds.
Mechanical sounds: There are none, except for the Turbo, no exhaust pop, no brake squealing, no gear whine, no shift sound, nothing, quite a bummer in cars like the GT LM.
Wind: Almost the same as in GT4, i like it. When you drive in the cockpit view it differs from car to car a bit.
Environmental sounds and effects: Great, driving through a tunnel changes the sound very realistic. The echo which you can hear in the replays is also perfect. Curb sound is like in GT4 which is anything but bad. The sound from the spectators is also GT4 style plus better sound effects (and you dont notice the loop restarts so fast anymore at a camera change in the replays or while driving).
The sound quality is also good and it seems to have a perfect 5.1 and 7.1 mix (uncompressed, Hip-Hip Blu-raaay).
If the engines would be more realistic everything would be fine, but hearing cars like the BMW M3 just makes you sad, it takes away so much from the overall feeling. It looks like a TV broadcast but sounds like a PS2 game, thats just wrong.
EDIT: Yes, the tire sounds itself are spot on, the only wrong things about them are that theyre too loud on racing tires and they sound always the same, while wheelspin, while breaking, while sliding and so on.