if you want good audio than you need good sound card, the best from creative, chose one affordable for your pocket,
get the speakers from Creative too, they seem to make it very good for computers. My all family has Creative and lets face it the sound is amazing, but as long as Creative makes the best sound cards (in my opinion and many people i know) you still can get better speakers but creative in wood should give you all you need for computer. Plastic or any other material is worst than wood, so pick wooden built speakers if you can afford them
other thing that might help is reinstalling drivers for your sound card, try get the newest from the manufacturer,
try reinstalling direct x, as it is responsible for sound and graphic on PC (I believe)
and check your speakers for example plug them in to mp3 player to check for sure that it is sound card or software not speakers,
you can also try to think if before trouble appeared if it did, if you installed some codecs, if so, unistall them.