If you really want to know about what sound card and speakers are best for use with your computer, visit The PC Hardware Blue Book , but to properly check your MP3s for sound purity you should have some decent sound equipment. If you have a portable stereo or stereo equipment that your not using on a regular basis, you should definately use that instead of the puny little computer speakers that most people use. I'll never go back to those crappy old Labtec speakers I used to use. An unshilded speaker near your magnetic media will likewise cause data loss. By close, I mean 3 to 6 inches.
Subwoofers kick ass.
Corner positions are best, as subs are non-directional. Mel Tsai's Portable MP3 Player Let's face it. Portable CD players have their problems. They skip because of the slightest bump or shake. Sure there's anti-skip technology, which basically loads the music a few seconds ahead into short term memory so that what you hear is from a memory chip and not the CD, but keeping that running all the time drains the batteries even faster than the blazing speed it normally does.