I can burn audio CDs and listen to them on my PC, but my portable CD player and car stereo can't recognize them.
Commercial CDs are literally stamped from extruded masters, creating pits and lands that CD players have been designed to read. Burned CD, however, create darkened areas that mimic the pits and lands of commercial CDs. As you've noticed, not all players-especially older ones-can deal with these kinds of discs. But all is not lost. Your best bet is to burn at a slower speed, 4x or below. This creates slightly more prominent differences between the burned and non-burned areas. If you have a Plextor burner, though, you have an even better option. The bundled Plex Tools includes VariRec, which allows you to subtly alter the burning strength of your optical drives laser. By experimenting with various discs and burning strengths, you will almost certainly find the right combination for your CD player. VariRec is supported by both Nero and Easy CD Creator.