Is the sound "card" is not a card, but on a motherboard. Such affairs are low cost options and certain no attention is paid to any frequency response curve since the amplified speakers are likely to be a bundled deal as well.
In a line, "that ain't hi-fi."
Be sure to look at all the sound controls in the control panel to be sure someone didn't diddle with the base/treble controls. Some have them, some don't. New Windows users just need to be shown where these are, but sometimes you can't because like most things, the vendors can modify such for more eye-candy. Here' s a typical picture which may or may not show the bass/treble control.
In closing, some MP3 players offer a EQUALIZER and I suggest you try Winamp since it has it. http://www.winamp.com
A funny equalizer I tripped over was at http://www.tweak3d.net/tweak/minisound/2.shtml
I have a "stigma tel C major" sound card (i think) and the sound is very dodgey for when i listen to music the voice of a person is very faint and the bass tune is very loud,, the speakers aren't that old either