For some folks, a lot of sound and/or bass is "good sound". For others, accurate sounds qualify as "good sound". I'm going to guess that's where the confusion comes in: no amount of high-price audio equipment will make the C6 Corvette race cars sound like they're powered by anything other than the wheezy 3.8L GM V6.
Out of all the modern competition (Forza, PCARS, AC, Driveclub), I can't think of a game that sounds worse than GT (in terms of engine samples — the audio engine is actually quite good). If your own personal bar of acceptability is set low enough that GT is "good", that's fine. It's your opinion, and you're entitled to it. But it doesn't invalidate the opinions of others who believe it should improve, or that, in its current format, is unacceptable.