Let's start by taking a look at speakers. These are perhaps the easiest way to improve your in car audio quality, since they produce the actual sound within the confines of your car.
While your standard speakers often try to produce bass, treble, and midrange sounds all with one unit, aftermarket speakers often feature multiple components like tweeters and woofers that produce their own sounds. You get a higher quality sound and a more pleasing, distinct audio range this way.
Most factory speakers come with a flimsy paper cone and cheap foam materials that don't produce deep, lasting sound. They wear down over time due to continuous use, ambient dampness and even the sun's UV rays help degrade some speakers. Thanks to these factors, the quality of the sound drops. Replacement speakers can come with cones made of Kevlar, rubber, polymer composites (like polypropylene), Mylar film and a whole host of other materials [source: Yoder]. But keep in mind, the most durable and lasting materials aren't necessarily the ones that produce the best car audio sound. Speakers made of rubber provide excellent sound as well as longevity; foam and cloth units cost less, yet still offer great performance [source: Nail].
A quality set of speakers can be had for a few hundred dollars, but you should know that there are extremely complex and pricy ones out there, too. That is, if you have the cash. Just remember: more expensive doesn't always mean better sound.
Speaking of sound quality -- just how much does external noise detract from your overall listening experience? Is there anything you can do about it?