From physics side, we have more low-power sound projectors directed straight to listener, thus we hear more direct sound, than reflections, and that's equal better sound quality.
There have been very few innovations in sound reinfocement for a many years, it's the same technologies invented in the middle of 20th century.
Just to name a few innovations - better materials for loudspeakers (like adding kevlar to woofers, ribbon tweeters in audio monitors), widespread of line arrays and horn subwoofers in proaudio market, and now AD/DA converters become really cheap and good, so we have digital audio with adequate quality everythere - ipods, integrated audiocards, tvs, cars.
As for cars - it always been shittiest loudspeakers installed by default, bought in China by weight. And as loudspeakers have good quality (frequency response, sensitivity), you have not much to improve, except their location and splitting up frequency range for more bands, thus reducing distortions brought by individual ls, all that followed by increased quantity of loudspeakers.
But all car owners i met measure sound quality with speed your brain turns in jelly, from all that subwoofers in trunk.
