This thread is a good idea and has my support. But if you are going to do an official pettition you need to be way more organized than this. Point out the biggest dissapointments, point out the best sound samples, point out a feasable replacement for a bad sound sample and do it all on a legible organized maner.
Sound issues:
1. Inconsistent and often inaccurate / incorrect allocation of sound samples to cars.
Many cars have sounds samples attributed to them that do not reflect the actual sound that car would make, or even a sound that an engine of the same configuration would make. This is most true of the modified sounds, i.e. sports, semi-racing and full racing exhausts. Many cars that sound fine in stock form are "ruined" when modified, but sadly there are a number of stock (or racing cars) in the game that fall wide of the mark, too.
2. Questionable quality samples
Many samples have audible looping artefacts (some persisting since GT3, or earlier) which are a real immersion breaker. Others are a mess of indistinct noise due to instabilities in the recording (e.g. fuel distribution in racing cars), or layers of disparate sounds merged together to form a more "dynamic", but unfortunately less convincing sound overall. Some may argue that the collision sounds fall into this category, but perhaps there just needs to be more variety.
3. Missing details from the sounds
Intake sound is very rarely audible in the cars in GT5, and has been so since the beginning. Intake sound, for naturally aspirated cars in particular, is a major contributor to the overall sound, and is particularly important in the interior. Without it, many cars have only the whirry and thin mechanical sounds coming from the engine compartment, which is clearly unappealing and contributes the most to the "vacuum cleaner" reputation. Additionally, changing the intake parts on a car makes no difference to the car's sound, which it would normally do in real life without substantial effort to prevent it.
Other missing sounds include superchargers, and other miscellaneous incidental sounds you would normally hear in the cockpit of a car, or as an external spectator. Turbo sounds are also inconsistent and often inaudible, as are the overrun pops - both could do with a boost in volume in many cases, where appropriate.
4. Low-volume reverb
Reverbs can add a sense of space and can really help put the player in the game, but whilst in GT5 they are usually very detailed, and quite long-lasting, they are often very low in volume relative to the cars themselves. I.e. the "wet / dry mix" is usually far too "dry" in all situations, which leads to a sense of sterility in the overall soundscape. The mix should scale to be more "wet" as a car drives into the distance, and / or as it is occluded by objects and scenery, for a more convincing effect.
5. Too few sampling points in RPM range
Whilst this is understood to be because extra samples would require more memory than can be spared, the low number of samples used across the RPM range contributes to the thin sounds in the higher engine speeds. Combined with the whizzy nature of the "engine" samples, this probably also contributes to the "vacuum cleaner" legend.
6. Sound mix is highly hardware dependent.
The standard stereo mix is known to be lacking in comparison to the multi-channel mix. It lacks spatial definition and can often sound a bit muddy.
Additionally, there is no support for virtual stereo surround natively, particularly useful for users of stereo speakers or headphones - this sort of thing is exactly what Sony specialise in.
Finally, in order to get the correct impression of bass in GT5, it is required to have dedicated bass hardware (sub-woofers) or to fiddle with equalisers to achieve the desired effect. It may be useful to investigate some tricks (again, Sony stuff) to counter this for those users without dedicated bass reproduction, or the ability to adjust the frequency response of their hardware. This "lack" of bass also contributes to the "vacuum cleaner" reputation.
Just a start, adding examples, as appropriate, to these categories would further its usefulness, if it is useful at all.
We should also include a list of all the stuff that is great about the sound.
But as I see, as more brains we have here, as more reasonable solutions we get! 
