There's only so much you can do. A car is filled with funny angles, plastics, and a lot of glass. It's never going to be a particularly good acoustic environment even if you manage to shield 100% of the road noise.

What other effects does that have? Weight, heat, etc.

> I recommend ripping your car apart and putting sound insulation panels everywhere you can! Also replacing your sound system...

Ripping the car apart to install sound insulation might void your warranty.

I have a Bose sound system in my car with 11 or 13 speakers (they're everywhere), but the car is so poorly insulated that I have to turn up the volume quite a lot at highway speeds, or even after about ~ 35-40MPH.

Or you can just embrace the 128kbps lifestyle. :D

That's the point, isn't it? You'd have to spend a lot of money on related stuff before audio upgrades, otherwise those audio upgrades are just money wasted.