Onboard sound on my wife's emachine was a bit weak with evidence of some background noise on quiet tracks. Installed a cheap creative live 5.1 card (ebay)which cured it and made a big difference plus the ability to upgrade to 5.1
GANDALF <|:-)> is right that you need good quality speakers. Remember though, that it is the quality of the speakers that matters not the number. More does not mean better, no 5.1 or 7.1 pc speaker setup is ever going to sound as good as a pair of high quality hi-fi stereo speakers, but none will sound any good if driven by a substandard source.