Sound is a multipart situation, the quality of sound you hear is governed by the sample rate, number of channels, sound driver limitations, sound app being used, the sound card limitations, and the quality of speakers. You have not provided enough information for an educated answer.
Here is somethings we should know before answering:
1) what distribution of linux.
2) are you using standard linux soundconfig or
the much improved ALSA multimedia interface.
3) what chipset are you using for the onboard
sound
4) which player frontend are you using (there are over 150 players for linux with varying degrees of performance)
5) what are the capabilities of your current sound card
By varying any of the aboveyou can attain radical changes in performance. But without further info I cannot adequately give direct advice.
For example: A sis 7100 chipset onboard sound works crappy with standard soundconfig but excellant with ALSA. Using cdplay as the frontend works well with soundconfig but marginal with ALSA . The audioorchestrator app works well with both on highend speakers and high end sound card but sorely lacks if speakers and soundcard are low end.
So you see more info would be helpful.rick