Some cars have very noisy interiors that make good systems pointless. Most of your economy priced cars are in this category, with some VW's and Chevy Cruze being exceptions.
With the mid size sedans the sound deadening and audio systems takes a big step forward. Camry, Accord, Altima, Malibu, etc. Not a universal truth, but most of them. Upper tier Toyota or Lexus get really quiet according to consumer reports magazine.
But the Hyundai Genesis has been turning heads in this department. Whisper quiet and a stock Lexicon audio system with 14 or 17 speakers, but you have to get more than just the base model for the Lexicon(they do the audio for Rolls Royce and this one is a 7.1 dvd audio). A one or two yr old v6 sedan can be had for low twenties, and the options it comes with is impressive. I'm hoping it's my next car!