I think most people complaining about sounds in GT5 are ones that perceive sounds from movies and arcade racing games to be true to life. That is not the case for the most part.
I agree that, when tuned, some cars sound bad in GT5, mostly V8s. However, out of 4 different exhuast, atleast one does sound good. The stock exhaust is pretty much spot on. If you want a different exhaust (more volume), atleast one of the performance exhausts sound good for the most part. If you fully tune every car, you probably won't like the exhaust. It is a flaw in their system. It may be corrected with better equipment in the future, but for now, GT5 has the best and most accurate stock exhaust for a console game available, except for PGR4. I install which ever exhaust sounds the best and most accurate. If I leave 20 hp on the table, so be it, I will just have to drive a little better.
The tire sounds, wind, etc... is all pretty good as is also. They have toned down the transmission whine, but it is still louder than I would like using stock exhaust, but it is pretty accurate.
I like the improved sounds!