no sound card.. what do i do? which one? how to???

i got a used computer. its a dimension xps r400. i reinstalled windows 98
after i brought it home. the speakers are hooked up right and i can hear a
drum sound, and thats all. i dont think theres a sound card. when i try to
play back, this is what it says: cannot playback the audio stream, no
audio is available or hardware is not responding. then it says there is no
sound card. please help, do i need a sound driver and a card??? im new at
this computer stuff. thanks so much